If under attack, any country has the right to repel the attack, according to
international law. But the right of self-defense is not unlimited. The
standard precedent is the Caroline case, which held that action in
self-defense should be confined to cases in which the "necessity of that
self-defense is instant, overwhelming, and leaving no choice of means, and no
moment for deliberation." Thus, self defense would permit the United States to
shoot down attacking enemy planes, but not to wage a war half way around the
globe a month after a terrorist attack, a war that U.S. officials say might go
on for years. Instead, this is the sort of situation that should be turned
over to the United Nations for action.

But let's suppose someone doesn't like the above formulation. What norm would
we want instead? If a country's civilian population is attacked, then that
country has the right to determine the perpetrator to its own satisfaction,
issue an ultimatum, determine on its own the adequacy of the response to the
ultimatum, and attack the perpetrator's host country, causing great civilian
harm. Would we really want this to be a universal norm? This would mean that
Cubans could attack Washington on grounds that Miami harbors support for
terrorists who have attacked Cuban civilians. Likewise, Iraqis, Serbs, and now
Afghans, not to mention Vietnamese, Laotians, Cambodians, Colombians,
Guatemalans, and so on, could all target Washington on grounds that the U.S.
government has attacked or abetted attacks on their civilian populations -
and, for that matter, ironically, Washington can attack itself, on the grounds
that it abetted the creation and arming of bin Laden's terror network which in
turn attacked the U.S.