United States Congress passed the War Powers Act in 1973, when it was believed that several previous presidents, including John F. Kennedy, Lyndon Johnson and Richard Nixon (who was still the president at the time) exceeded their authority when they sent troops to Vietnam without congressional approval.
The Constitution places the authority to declare war squarely in the hands of Congress, not the president. The Vietnam war was never declared.
The War Powers Act itself requires U.S. forces to be withdrawn from foreign lands where they're involved in hostilities in 60 days unless Congress ratifies the deployment. The president may seek a 30-day extension if that's what's needed to withdraw troops. The president is also required to report to Congress, in writing, within 48 hours of committing troops abroad. Within the 60 to 90-day window, Congress may order the immediate withdrawal of forces by passing a concurrent resolution, which would not be subject to a presidential veto.
On Oct. 12, 1973, the U.S. House of Representatives passed the bill by a vote of 238 to 123, or three votes short of the two-thirds requirement to override a presidential veto. There were 73 abstentions. The Senate had approved the measure two days earlier, by a veto-proof vote of 75 to 20.
On Oct. 24, Nixon vetoed the original War Powers Act, saying it imposed "unconstitutional and dangerous" restrictions on the president's authority and that it would "seriously undermine this nation's ability to act decisively and convincingly in times of international crisis."
But Nixon was a weakened president--weakened by his abuse of authority in Southeast Asia, where he had dispatched American troops to Cambodia--and of course kept American troops in Vietnam--without congressional authorization, long after the war had become unpopular and was clearly lost.
The U.S. House and Senate overrode Nixon's veto on Nov. 7. The House voted first, and passed it 284 to 135, or with four votes more than required to override. There were 198 Democrats and 86 Republicans voting for the resolution; 32 Democrats and 135 Republicans voted against, with 15 abstentions and one vacancy. One of the Republicans voting against was Gerald Ford, who said the bill had "the potential for disaster." Ford would be president within the year.
The Senate vote was similar to its first, with 75 to 18, including 50 Democrats and 25 Republicans for, and three Democrats and 15 Republicans against.