The 1973 War Powers Resolution passed by Congress (over Nixon's veto) requires the President to notify Congress of any deployment of troops to military action and sets a 60-day limit on action without Congressional approval. This set the atmosphere in Congress during the 1975 Spring Offensive, as Congress was in no mood to allow Ford to deploy offensive air assets to aid the South Vietnamese, even though Nixon had secretly promised Thieu, in order to get him to the table at the Paris Peace Accords.

However, even though American air support could have seriously dented the PAVN advance, it was never seriously considered as an option—if Congress would never allow it and the American public was strongly against it, there was no point in keeping it as an option. The South Vietnamese, much to their misfortune, held out for American support up until the end. Ford however did request emergency aid to be sent to South Vietnam, which was denied by Congress.

As for why the North Vietnamese did not invade South Vietnam earlier, well they did—the 1972 Easter Offensive was a large-scale conventional offensive, aimed towards striking at the heart of South Vietnam and quickly seizing the capitol, Saigon. However, it failed because American air power was able to blunt the attacks while the South Vietnamese forces on the ground recaptured territory. The reason why another offensive did not take place until 1975 is because the PAVN had sustained heavy losses, both in terms of equipment like tanks, and in manpower.