As I recall, all of the US and allied troops were out of Vietnam long before the North Vietnamese invasion and the fall of Saigon. Congress refused to increase the military aid package for that fiscal year, so the South Vietnamese army basically had no rifle ammunition, no bombs to drop on the tanks coming south on QL-1, and so forth. I suppose one could say that Congress ended that war - by handing a military victory to the North. Quite an example to emulate.