Every 20 years.
The US finds a war to fight about every twenty years. All of them are fought in the name of peace.
It's sort of like space exploration and funding massive telescopes. It's all done in the name of finding life on Mars, Europa, or someplace else in the Universe.
You are right, all that's needed is some sexy reason such as 9-11. Where's George W. Bush when we need him, his vision, his courageous leadership, his Three Wise Men, Cheney, Rumsfeld and Rice?