What if the US loses the Cold War in a Britain-esque way...we simply can't afford to keep at the top?
(literally, in terms of finances; like the UK after WW2, no longer at the top of the world)
This idea came to me when I wondered "would 9/11 have been directed against the Soviets if they were the lone superpower?"
...but I didn't want the US collapsing like the OTL USSR did, so I figured a bit of money problems might force us to settle for second place (we'd still be higher than everyone else, at least initially)