The U.S. had been supporting South Vietnam to defend its independence since 1954. As in President Johnson’s words, to leave South Vietnam to its fate would undervalue America’s words & commitment and ultimately shake its credibility in the world. The U.S., therefore, was obligated to keep their promise of helping South Vietnamese people in their war against the North communists. They had fought hard for a “noble cause”.
Gradual withdrawal of U.S. ground forces began as part of "Vietnamization", which aimed to end American involvement in the war while transferring the task of fighting the communists to the South Vietnamese themselves and began the task of modernizing their armed forces.
