The Times (London) REUNION, A Memoir, By Tom Hayden, Hamish Hamilton, £17.95 Wordsworth wrote of the Happy Warrior. Tom Hayden can reasonably claim to be the Happy Student Radical, American version, what each of his innumerable 1960s confederates might have wished to be. Hayden wrote, in 1962 at the age of 22, the manifesto of Students for a Democratic Society, perhaps the key institution of the just-emerging New Left. He was an 'outside agitator' in the still-segregated South, one of the only two whites to be jailed in Martin Luther King's first large-scale campaign of civil disobedience. He 'organized' in the black ghetto of the northern city of Newark, New Jersey, which promptly erupted in the worst race riot of the decade. He went to Hanoi as a 'peace' campaigner, returning triumphantly with American prisoners of war in tow. He was one of the 'Chicago Seven' convicted in a sensational trial of conspiring to disrupt the 1968 Democratic Convention.