Nixon was forced to resign because of Watergate. His plan was “Vietnamization” of the war, withdrawing U.S. troops while promising to resupply South Vietnamese forces with equipment.
Ho Chi Minh’s plan was to continue the war while at the negotiating table. He figured on five more years, and withheld a full blown invasion of the south because he thought that would cause Nixon to unleash a furious bombing campaign.
But Nixon was out, Ford was in, and when it came time to appropriate funds to resupply South Vietnamese forces, Congress refused.
Ho Chi Minh sped up his timetable. As the North Vietnamese Army approached Saigon, the U.S. Ambassador offered to evacuate a high South Vietnamese official to safety in the U.S. He declined, saying that he would remain with his people and expressing sadness and surprise that the U.S. would so readily abandon his nation and the promises they had made. This story is recounted in the Michael Lind book.
A priest of our diocese (now) was at that time in seminary in South Vietnam. When the NVA took over the South, the seminaries were closed and seminarians sent to reeducation camps. During the next several years, he made several unsuccessful attempts to escape the country, but was caught and imprisoned each time. A third attempt was successful. A loss for Vietnam, a gain for our diocese.