All US presidents sent men to South Vietnam to obtain situation reports.
President Johnson asked congress for and received a resolution giving him power to use whatever was necessary to protect U.S. interests in Vietnam on August 7, 1964.
Army General William Westmoreland, who was the supreme commmander of all U.S. forces in Vietnam, asked Johnson in 1965 for more troops. This widened the war and the first major battle in Vietnam was the Battle of Ia Drang in the Pleiku Province of South Vietnam.
The US couldn't declare a war in Vietnam, that would've implied total war. Total war meant deploying atomic weapons...that would've caused WWIII. The whole purpose of the cold war was to avoid an atomic war...Vietnam was a fuse that could have led to an atomic explosion...a fuse that world leaders wanted to ensure didn't detonate.
The North Vietnam army was invading South Vietnam and Sv asked the US for help in repeling the Communist invasion.
Kennedy was asked to send additional troops to Vietnam. He sent additional troops and military advisors over to Vietnam to help.
Because the US asked them for assistance.
The US asked for their help.
Invitation by the Republic of South Vietnam.
We asked, they came.
Yes they did.
President Lyndon Johnson asked Chief Justice Earls Warren to investigate the assassination of President Kennedy.