Reagan assassinated by John Hinkley during Presidential campaign. Carter wins re-election over George Bush despite the bad economy because Bush didn't have a positive vision.
Carter disarms in the wake of Soviet expansionism. Radical Islam is subverted by the Soviets as the Vietcong was to the North Vietnamese militarism. The Middle East falls to Islamic-Socalists movements, and the U.S. can't obtain oil from the region.
U.S. Oil production blocked by radicalized anti-growth groups in and out of the administration. U.S. defense funding cut. Authoritarian Left takes over do deal with economic collapse. Republican Party suppressed. Carter makes move to suspend 1984 elections. U.S. Military attempts to defend Constitution results in Soviet Army forces invited into U.S. to prop up Carter Regime. Carter ouster for more radical Mondale-Hart Administration.
Okay, thanks for your Red Dawn paranoid fantasies. I agree that Mad Bad Rabbit's scenario would make the most sense for a weakened USA. The other scenarios don't really harm the USA so much as lift up other powers.