To answer your second question: NATO (North American Treaty Organization, which the US is a member of, you don't have to name them separately.) shouldn't do a damn thing.
As much as Osama was a bad man, he was just that: One man. Last time two nations went to war over one man was in 1914 when Archduke Franz Ferdinand of Austria was assassinated.
I think a quote from the West Wing episode "Twenty-Five" puts it best:
You don't go to war over one man. Last time that happened, 38,880,500 (est.) died in World War One.Speaker Walken: Franz Ferdinand, who was the nephew of the Austro-Hungarian emperor, was killed by a group called the Black Hand. And because they were a Serbian nationalist society, the empire declared war on Serbia. Then Russia, which was bound by a treaty, was forced to mobilize which meant that Germany had to declare war on Russia. Then France declared war on Germany, and that was World War I. Because the emperor's nephew was killed.