welcome to "the whole story." i'm anderson cooper in tel aviv. the october 7th attack by hamas, which the u.s. labels a terrorist organization, left at least 1,400 people dead. more than 200 people are being held hostage according to israeli officials, and many more people are still unaccounted for. in response, israel began a bombing campaign in gaza. according to the hamas controlled health ministry, more than 4,000 people have died there so far. the story of hamas is complex. its roots date back to the formation of israel in 1948. itit has many factions includin the so-called military wing, which perpetrated the terror attack. over the next hour, cnn's sara sidner brings us the whole story on hamas from how they were formed, what their ideology is, and how they have evolved over the decades. we want to warn you some of the images you'll see in this hour may be disturbing. >> pure unadulterated evil. >> i can't even try to comprehend these monsters. >> terrorists. >> this is not islam. >> they should be spit out. >> we cannot expect that israel will continue to blockade gaza. deprive them of their freedom and assume that they will not at some point resist. at some point it is going to boil over. >> october 7th was different. october 7th changed everything. >> what happened on october 7th was absolutely unprecedented. hamas has certainly targeted israeli civilians before. that's not new. what is new is simply the scale and just the ruthlessness. >> reporter: how do you define hamas? who and what is hamas? >> hamas primarily is a social, religious, political movement. >> hamas is seen by most palestinians as part of their social fabric. there are plenty of palestinians who cannot stand hamas, but they recognize that some of their neighbors, some of their family members, are hamas supporters. hamas has several different aspects to it. it certainly does have a military wing, but it also has a political wing. it has social services. it sees itself as a movement. it calls itself the islamic resistance movement, and the movement is a comprehensive one. >> hamas is a nationalist movement that's committed to the notion of armed struggle for the liberation of palestine. out of all the different palestinian factions that exist, hamas is the only party that has an organized military and a very well-resourced military force. so, what that means is that for many palestinians, hamas is the only party that can actually defend palestinian civilians against israeli aggression. >> that's the reality. that is how many palestinians view and consider them, a line of defense. >> since 2007, hamas has governed the gaza strip, a 25-mile-long, 7-mile-wide stretch of land with more than 2 million people largely cut off from the world by an israeli blockade, an area humanitarian rights groups have called an open air prison, but the rise of hamas and what led up to its massacre of more than a thousand men, women, and children in israel is a story that begins long before hamas ever existed. >> this did not start on october 7th. ♪ >> reinforcements disembarked to help in the protection of the holy land from disorder. >> the british had a mandate to govern palestine after world war i, and it said a couple things. it said you should prepare the territory for independence, but you should also facilitate a jewish national home. >> the british try various mechanisms to make them agree on some kind of settlement. >> and the british found they couldn't do both, so they gave up. they just left after world war ii, and they handed the problem over to the u.n. and said, we can't deal with it. >> the palestine problem moves into another stage of discussion. >> the u.n. sets up a commission that comes to the conclusion the best solution here is to take this land and divide it, roughly speaking, half to the palestinians, half to the israelis. the jews accept that deal. the arabs do not. >> and so war broke out. >> arabs and jewish nationals fought each other bitterly and relentlessly. >> as a result of that war, the state of israel declared itself and took control not simply of the territory that the u.n. had allotted to it, but some other territory as well, a lot of which had significant palestinian population. >> the new jewish state, israel, was born in a bath of blood. >> the birth of the state of israel for palestinians is called the -- [ speaking non-english ] which means the catastrophe, because in order to pave the way to establish israel as a jewish state, there needed to be a mass ethnic cleansing of palestinians. more than 700,000 palestinians fled outside of the land of palestine, and palestine was essentially decimated. >> women flee with what belongings they can carry. >> and israel said essentially we're not allowing them to return. you may have thought you were leaving for a week, but you're never coming back. >> by the end of 1948, a newly formed israel had claimed 78% of the land of historic palestine. the palestinians, who hadn't fled to neighboring countries, settled in the remaining 22% of land that israel had yet to conquer. >> what is left is the west bank and gaza. so the egyptians control gaza. the jordanians control the west bank, and that's how it was until the 1967 war when arab armies are massed on israel's border. >> the frontier of the gaza strip just a mere kilometer away. >> 1967 is one of the biggest historic moments in the middle east. israel launches a surprise attack against egypt. >> israeli forces have routed the armed might of their arab neighbors. >> the israelis conquer the west bank and gaza, and this is a moment of great exhilaration in israel, because they feel as though they have defeated the arab armies in an extraordinary military success. they have not figured out what they're going to do with the millions of palestinians on that land. >> after the six-day war, the millions who fled to gaza and the west bank in 1948 are officially under israeli occupation. >> which left all these people in gaza and the west bank citizens of nowhere. they weren't citizens of the israeli state, and so they were just stuck. >> palestinians began to say, we need to liberate ourselves. >> throughout the '70s and '80s bursts of violence between palestinians and israelis were commonplace. the palestinian liberation organization, which is formally recognized by the world, was led by yasser arafat who was operating in exile outside the palestinian territories, but inside israeli-occupied gaza, a new resistance movement was under way. >> the first intefadeh was one of the biggest mobilizations of palestinian civil society and actors against the military occupation, and the idea was they would disrupt the occupation apparatus, so they refused to open shops. they blocked highways. they refused to give tax. it was a period of significant agitation. >> reporter: around the same time islamists within gaza and the west bank were shifting focus to a more aggressive approach towards their israeli occupiers, and a new armed resistance group would emerge officially known as hamas. >> the spiritual leader in the gaza strip was one of the earliest founders and leaders of the muslim brotherhood chapters in palestine. when hamas was established in 1987, he emerged as the leader of the movement and in some ways the spiritual guide of the movement. >> reporter: unlike the secular palestinian resistance known as fatah, the newly established hamas was not interested in liberating gaza and the west bank alone. instead, it set out to eliminate the state of israel altogether as outlined in its 1988 charter, a goal that made hamas an outsider to any peace negotiations, including the oslo accords. >> the oslo accords really set off earthquakes in both palestinian and israeli society. >> as our wars have been long, so must our healing be swift. >> what the oslo accords ultimately resulted in was that the plo recognize the state of israel and so conceded 78% of the land of historic palestine, and in return the israeli government recognized the plo as the sole legitimate representative of the palestinian people. there was jubilation globally, and many palestinians, despite this historic concession, believed that this might pave the way to the establishment of a state on 22% of their land, but this was by no means noncontroversial or unchallenged, and for hamas specifically, this was something that they were fundamentally opposed to. >> those negotiations never really got off the ground, and one of the reasons was this upsurge of violence. >> one of the deadliest attacks to derail peace came in february of 1994 just months after the signing and historic white house photo-op. >> a jewish settler entered the mosque while muslims were worshiping there. the dozens of dead and scores of injured were rushed to hospitals in nearby towns. >> that was a turning point, because hamas then decided to begin employing suicide bombing as a form of resistance. >> reporter: 41 days after the mosque attack, hamas responded, detonating its first lethal suicide attack, killing seven israelis at a bus stop. >> the big reason oslo failed was violence. the perception on both sides is that their adversary is not serious, israelis saying arafat is not completely stopping terrorism, palestinians say the israelis are dragging their feet on a pullout. they are increasing settlement building even as they are making promises they're going to leave. >> the fundamental reason hamas has gained strength is that the palestinian authority, which is their competition, has been seen as feckless, corrupt, and unable to deliver on its core promise, which was a palestinian state. >> reporter: but to the west hamas was not a negotiating partner. instead, it became a threat. in 1997 the u.s. officially designated hamas a foreign terrorist organization. >> if you protest nonviolently and so on, you are shot at. you are imprisoned, and so they left only one avenue actually, which is armed resistance. >> hamas has decided that it is going to use violence. it is going to use violence against civilians. it is going to be brutal. >> reporter: over the next decade, hamas continued its violence, and it would meet more violence in return. will they go home with a deal? the latest from camp david. >> this wooded maryland retreat of u.s. presidents, camp david, was the site of a potentially historic summit in the summer of 2000. >> you had yasser arafat, the legendary leader of the palestinian liberation organization, who had a lot of credibility with palestinians. you had d ehud barakak, very distinguished israeli military figure, now prime minister, and you had bill clinton. >> reporter: their goal? end decades of hostilities, forge a palestinian/israeli peace accord. >> if they cannot make progress now,w, there will be morore hoststility and more bitterness >> it seemed as though they had kind of come to an agreement, and then arafat pulls out at the last minute,e, best we c can te because e he believeved that if did d this, hamamas would gagai power. >> so, the israeli story on camp david is that the israeli prime minister offered significant concessions, and the palestinians said no. the paleststinian viewew was th they were set up at camp david, that israel and the united states made offers they knew that yasser arafat could not accept, and as a result they were made to look bad. >> reporter: just two months after those talks failed to get an agreement, a former israeli defense minister, who many arabs called the butcher of beirut, for israel's 1982 invasion of lebanon made a provocative, heavily guarded visit to the temple mount. a holy site in christianity, judaism, and islam. it is also a flash point of contention over who should control it, israelis or arabs? >> the violence that observers believe to be inevitable erupted. >> reporter: just the first sparks of what became the second intefadeh, a violent and deadly conflict between israelis and palestinians over israeli occupation of the west bank and gaza. the second intefadeh would last nearly five years, a half decade fight in which hamas became known for their routine use of both terror and destruction. >> over time more and more of israel came into range. also the number of rockets and missiles increased. >> reporter: israeli forces demolished over 4,000 palestinian homes and arrested thousands. israel shut down and bombed ministries and infrastructure trying to coerce palestinian leaders to end the violence. between 2000 and 2005, there were over 4,300 registered fatalities, with a palestinian to israeli ratio of just over 3-1. finally, in february 2005 came this announcement from the palestinian authority and israel. >> we have agreed with the prime minister ariel sharon to cease all acts of violence. >> reporter: later that year, israel unilaterally began implementing its so-called disengagement plan to evacuate israel's settlements and military posts from gaza and a section of the west bank. thousands of settlement residents s lost theirir homes. > it was boboth a physisical also a p psychologicical disesengagement t on behalf f o israelis from gaza. it was no longer their problem. >> it was a very, very difficult, painful step for israel to take, to pull settlers out, and how did palestinians respond? by s shooting rorockets, andnd is s some truth h to that. it's also o true, thouough, tha whenen israel pupulled out o of it continunued to envevelope ita blockade. >> reporter: a blockade to isolate hamas and attempt to prevent smuggling of weapons, but it also severely limited the transport of basic necessities for palestinian citizens in gaza. > the blockckade was ququite horrrrific. there was s an immediaiate coll in the quauality of lilife in g. medidicines, fooood items, w wa all of the items of a normal life were immediately suspended, and the immediate impact was a signgnificant inincrease in n p and d destitutioion. >> the years directltly after t israeli disengngagement frfrom israelel in 202005 leadingng upe palestiniaian elections in 2006 saw an unprecedented wave of violence. i remember being in the city where i was raising my young son and feeling unsafe to simply go out about my daily life in the streets. >> reporter: and from gaza, hamas continued to terrorize israel. >> so,o, israel wawas operatini under r the assumpmption despip mimillion palelestinians b bein imprisoneded, it couldld still expectct calm. whatat hamas wasas doing wasas this pereriod shattetering that illusision, so evevery few mono if not years, it would fire rockets in order to force israel to reconsider. >> reporter: in 2006, nearly a year after the announcement of the cease-fire, elections for the palestinian legislative council were held in gaza. >> this feted and bleak tunnel -- >> reporter: cnn's chief international correspondent christiane amanpour covered that election and its aftermath. >> it was very clear that that election was insisted upon by the administration of george w. bush and d condoleezzzza ricd that group of amerericans who o believed t that the iriraq war 2003 w would bringng democracyc ththe wider mimiddle east.t. this w was a very y flawed assusumption, anand the amereri insisteded on this e election, thoughgh the israeaelis and tht palestiniaian authoritity, the lelegitimatelyly internatitiona accepteded and recogognized paleststinian auththority, wara the ununited statetes not to l ththis electioion go aheadad, b they feared that hamas would prevail, and that is exactly what happened. >> reporter: the hamas victory was resounding, winning 76 of 132 seats in the legislature. shocking results for the u.s. and israeli officials, bringing to power a group the united states had designated as a terrorist organization, but a group that had been making a difference in the lives of every day palestinians. >> for many palestinians, hamas is a lifeline. for two decades they've built a grassroots network of affordable social services like this medical clinic that charges $2 a visit. >> t they providided educatitio. they were e very well l entrenc with the citizens and civilians, so that is one of the reasons why hamas s won in gazaza in 20. >> so o after the e 2006 electc anand hamas vivictory, thehere questionons on the scope of hah' powewer, is it s still underer palestinian authority, or is it independent, and there are questions about who runs particular parts of gaza. hamas is worried that there will be a coupe. as a result, it effectively does a coup. >> reporter: the coup in 2007 known as the battle of gaza was relatively brief, bloody, and left no doubt hamas was not a are part of the palestinian authority, which governed the west bank. hamas was a much more extreme and violent group now solely in charge of gaza. >> it seizes power in gaza in 2007, and it violently goes after the individuals associated with the palestinian authority. some are thrown from windows. many are arrested, tortured. >> but one thing is clear, hamas managed to do what fatah was unable to do prior to 2005, which isis to securere the stre in gaza, to put an end to the rampant lawlessness that we were seeing. >> reporter: having hamas in charge of gaza meant t the blockakade would c continue. >> so the cutoff of goods going in and out of gaza have a big impact on hamas. they make it hard for hamas to provide any prosperity to ordinary gazans. >> i visited gaza in 2012. you might have power for a couple hours a day, and these condnditions werere getting g w anand worse. inside t the gaza ststrip, hamas not t a popular r movement. ththere is no elecections. there's s no way to o get rid o them or anything like that, so people in the gaza strip just lived with them. >> reporter: and the conflict between israel and hamas would not abate, hamas attacking israel. >> that was the explosion. we just heard one explosion going off. i think it came from that direction over there. >> reporter: israel firing on gaza. you look to your left, destruction. i reported from both gaza and israel, telling stories of destruction and dismay. around 3:30 this morning here in gaza city, a massive explosion. we know there have been at least four bombardments, and it would only escalate in 2014 after the israel security agency identified hamas members as responsible for the kidnapping and killing of three israeli teenagers from a jewish settlement in the west bank. and weeks later, the abduction and murder of a palestinian teen whose body was found in jerusalem. outrage and anger on both sides would not be contained erupted in the 2014 gaza war. >> israel put together an operation, which consisted of about 5050 days withth 70,000 israeli call-ups for the defense forces. they went into gaza and on three differenent axes in n the north in the center, and the south that resulted in 66 israeli deaths of soldiers, 6 israeli citizens. u.n. eststimates 2,1,100 palestn and hahamas deathshs during tha same operation. it was after that that the israeli government really started taking a different look at hamas and what they were doing in the gaza strip. more than 2,100 palestinians were killed. more than 60 israeli soldiers were killed. >> going in in 2014 into gaza after that sustained bombing, you just have this huge sense of the utter scale of the destruction. it was really hard, near impossible to find anyone who genuinely supported what hamas had done. >> translator: i hope god won't let anyone taste our suffering. this woman in gaza said. >> in the west bank where the majority of palestinians lived, the year 2021 was a very bloody year. >> a flash point has definitely been the possible eviction of some palestinian families, some of which have been living there for generations. >> reporter: according to the u.n. since prime minister benjamin netanyahu's re-election in 2009, there were over 14,000 instances where palestinians were forcibly removed from their land by israeli 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