Why did America start the Vietnam War?
It didn't. There had been fighting between the communists and the French since the end of WW II. The conflict escalated when it turned into a proxy war between the US and the Soviets/Communist China.
the war was already going on when the US joined it
They didn't, France did.
Why is any war started? It's mostly about resources. Money. Stuff. There is sometimes cultural and religious conflict. I've been digging into the Pentagon Papers. Here a couple of passages that refer to the opium trade in Vietnam at the time: Page 4 of this document discusses the "French controlled monopolies on salt, alcohol, and opium..." The next page (5) discusses Binh Xuyen, the gang that evolved from the Black Flag pirates. The CIA agent, Edward Lansdale, cited how the gang's leader, Le Van Bay Vien, colluded with the French and Bao Dai to control gambling and prostitution. He ran an opium factory and ran a distribution system. Here is a Truman administration document discussing Vietnam. There is a section discussing foreign trade, part of which reads: "The central government has derived its chief income from custom fees, excise taxes and monopolies, of which the opium monopoly is the most lucrative." (page 185). ^^^ Some of this stuff is right in the open, but people don't seem to notice or care. The sentiment of the remarks made by Eisenhower are often really downplayed today, just like in the past. Wars often have several causes, but I always start with the economics. Many people downplay the economics or ignore it altogether. Here are some excerpts from President Eisenhower's Remarks on the Importance of Indochina at the Governors' Conference, August 4, 1953 President Eisenhower's Remarks at Governors' Conference, August 4, 1953, Public Papers of the Presidents, 1953, p. 540: "Now let us assume that we lose Indochina. If Indochina goes, several things happen right away. The Malayan peninsula, the last little bit of the end hanging on down there, would be scarcely defensible--and tin and tungsten that we so greatly value from that area would cease coming." AND "So, when the United States votes $400 million to help that war, we are not voting for a giveaway program. We are voting for the cheapest way that we can to prevent the occurrence of something that would be of the most terrible significance for the United States of America--our security, our power and ability to get certain things we need from the riches of the Indonesian territory, and from southeast Asia." Sources in comments.
The north started the war by invading the south. The Vietnam War began after WW2, when the Japanese were driven out. Having formerly been a French colony, the French determined to recover Vietnam. The opponents to French rule were primarily Communists and they consolidated opposition under the Communist Viet Minh. It isn't a well-know fact, but 90% of the funding for the French recovery of Vietnam was paid for by the United States. It was an American proxy war from the get go. The South retained its own government and resisted Communist incursions. America provided military aid on a limited scale until American-held airbases were attacked by Viet Cong sappers. The came the Gulf of Tonkin Incident, which is still controversial. This supposed attack by small patrol boats on an American destroyer prompted President Johnson to send American troops into South Vietnam and effectively begin the American phase of the ongoing Vietnam War.
it was a mistake and millions died still Yugoslavia was better it was just bombing raids
Americans did not. Read your history.