The United States has only lost two wars since its creation in 1776. The first one was the War of 1812. The United States declared war on the United Kingdom for a variety of reasons including possibly the loss of national pride caused Chesapeake-Leopard Affair and launched three attacks on British North America (Today’s Canada). The British considered this an expansionary move to annex Canada. All three were repelled by the British Army, local militias and First Nations support. It resulted in the White House and the Capitol building being burnt by British forces. It wasn’t that much of a loss for anyone really (more like a stalemate), because the U.S. did achieve many objectives but this is probably the closest to home it has got for the United States so it is worth mentioning.
The second war that the Untied States lost was the Vietnam War against the NVA and Viet Cong to prevent South Vietnam from going communist under the control of the North Vietnamese government which was backed by the USSR and the PRC. It was one of the most drawn out wars in modern history and resulted in the unification of Vietnam under a communist government.
Since the war was becoming increasingly unpopular at home, Nixon eventually pulled back all U.S. forces in 1973 and Saigon fell to the Communists in 1975. Communist governments also took power in Laos and Cambodia. The biggest disadvantage America had was that the press in the United States was free to report about the horrors of the war, but in North Vietnam it was all censored.
While America probably could have kept fighting, the war lost political backing and therefore ended.
0/8, did not even do research.