This is an intersting article on the Mumbai attacks and why from the terrorist’s perspective they were so effective. The key seesm to be that by splitting into small groups, each one carrying out multiple attacks, they overwhelmed the security forces who normally would have expected the attackers to sit in one place.

As the article says, this is more of a problem for the US than anywhere else:

America was the only country in the world that allowed foreigners who are legally visiting to buy a gun.