likely would cut off their business with north korea. but the retaliation could really hurt the united states as well. >> reporter: that leaves the most daunting option. a preemptive military strike. general walter sharp, who commanded forces in south korea, says that should be an option if the threat is imminent. but with more than 28,000 u.s. troops near the dmz and the south korean capital only 30 miles from the border, the blowback could be devastating. >> it could inflict a lot of damage on seoul because of all the artillery within the range of seoul, the number of missiles they could launch into south korea. they could cause a lot of damage. >> reporter: general sharp says the u.s. has to make sure kim jong un understands that any u.s. retaliation for an attack on south korea will be much worse than anything kim jong un fires southward. wolf? it goes beyond the north