On March 8, 1965, the first official US combat troops, consisting of 3500 Marines, landed in Da Nang, Vietnam. There had been of course an ever-increasing number of "advisers" on the ground since before the 1954 French defeat at Dien Bien Phu, but these Marines were the first Americans who were sent there in force with the sole purpose of rooting out the Viet Cong to protect "democracy" in the not-really-a-country of South Vietnam.
We all know how well that worked out: Not well.
Sidebar: We very easily could have been there eleven years earlier. I knew a guy who had fought as a Marine in the Korean War and who was subsequently stationed in Japan. He told me one time that when it became obvious that the French would be defeated at Dien Bien Phu, the US loaded his whole division onto transport ships and headed south. For about a week they hove to, just out of sight, off the coast of North Vietnam while they made preparations for an amphibious landing. Then, for no stated reason, they suddenly left and steamed back to Japan.
I still have no idea what that was all about.
Before you dismiss this as just some more barracks-room braggadocio, keep in mind that I heard this story about 1961, long before most people in the US had even heard of Vietnam, let alone be able to find it on a map.