It would be the failure to achieve a military victory. The Republic of South Vietnam was ultimately taken over by North Vietnam, this Vietnam experience became known as "the only war America ever lost."
The United States involvement in the Vietnam War is debatable and controversial. The U.S. invaded Vietnam and engaged in a war that was "unwinnable". Half a million U.S. troops and 700,000 South Vietnamese troops were no match for the 250,000 soldiers in North Vietnam, according to Robert F. Kennedy’s 1968 speech, published at ABC-CLIO.com.
Even if the U.S. sent more troops to support the South Vietnamese, the South Vietnamese wouldn’t be able to protect or sustain their post-war military successes. Their lands had been ravaged by war and they had few resources to rebuild their nation. War left the nation in ruins.
In history classes, especially as you get closer to college, you are usually expected to know why an event is important. So when you study terms (definitions) for a history exam, the best way to make the information stick is to understand the term in context, or understand how it relates to other terms and facts. Check out some tips on how to study history terms at About.com.