The way things look now probably nothing. Most of Europe and for that matter the rest of the world still believe that we are the problem in the world today, so they will all sing goombayah and light candles for the sick, dead and injured for a couple of days or so and then go on to blame America's actions in the war on terror and Iraq for causing the attacks. The U.N. will tell say "watch your step America" The Democrats will be blaming Bush and the Neo Cons actions for the attack and say that what we needed now is diplomacy and dialog, not anger and retaliation and so on and so forth.
The more I think about it the more I think we would probably respond with targeted conventional weapons.
We should nuke all of their major cities but probably won't, this problem won't go away, they will get nuclear weapons and they will use them.