The worst strategic mistake in American history

Al Gore has said Iraq "was the single worst strategic mistake in American history" My guesses for the single biggest strategic mistake are:

• Entering WWI
• Continuing to give war materials to Stalin after D-Day and so allowing him to conquer Eastern Europe.
• Not giving more support to the Whites in the Russian revolution.
• Allowing other states to develop nuclear weapons. (We should have declared a monopoly on them and bombed anyone who tried to build them. From a Machiavellian strategic viewpoint you have to agree that this would have been extremely militarily beneficial for America.)
• Not peacefully solving the issues that caused the civil war.

Update: Mangan and Bobvis have some more suggestions.