Maybe the real question is why we continue to defend Europe. The Soviet Union was a threat to the United States both physically, but most of all, ideologically. There were lots of people who were willing to die for the concept of world communism. We could not allow Western Europe to fall under the domination of the Soviets lest we find ourselves overewhelemed.

With the end of communism, the Russians have fallen back on Slavic nationalism. The only people generally willing to die for pan-Slavism are a few silly Serbs and occassional Belorusian. So we have nothing to fear from the Russians unless we convert them back into an enemy.

As for Iran, they are no direct threat to US. Their threat is indirect, through terrorism.

All this missile defense plan does (and I am a big supporter of a US based missile defense program) is further inflame tensions while allowing the Europeans to free load off of our defense budget. This is very very ill advised.