As to the question, OBL and Al Qaida declared war on the US. As leader of the opposing army, he was a valid military target. The attack was perfectly legal within the rules of war.
The correct parallel is the shootdown of Admiral Yamamoto's plane over the Solomons in april 43, after allied codebreakers found out his schedule.
The only country that can complain is Pakistan. Since the US violated their sovereignty, they may choose to consider the operation an act of war and declare war on the US. However, considering the mood of the US public over the fact that Pakistan harbored OBL for years, I don't think that would be a wise move. Afghanistan was invaded for a lot less in 2001....
There is really no issue.
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The more interesting issue for me is the fact that the Republicans who raised a stink over Obama not getting prior congressional approval before bombing Libya did not raise a peep over Obama invading Pakistan to exterminate OBL.
Has the Constitution changed?
