How did the U. S. role in the Vietnam War change from that of providing economic and military aid to others who were doing the fighting to an American war in which U. S. forces did most of the fighting?
The basic answer to this is that Vietnam became an American war in a gradual manner. The US did not go in intending to take over the conduct of the war. However, as the US lost faith in the South Vietnamese government and military, it took more and more control over the fighting of the war.
At first, the US meant only to give aid to the South Vietnamese. However, it eventually became clearer and clearer that the South could not win the war on their own. The South Vietnamese government clearly had a very hard time winning any loyalty on the part of the country’s people. The South Vietnamese army had poor morale and did not fight well. As these facts became more and more apparent, the level of US involvement grew.
The US believed that the war had to be won so that communism could be contained. Therefore, the US gradually became more involved. First, it stepped up its use of air power. Then it brought in ground troops to protect the air bases and other US military personnel in the country. Then it started to have the ground forces participate in offensive operations. Finally, it brought more and more of these forces in and eventually took nearly complete control of the war.