The expanded air war...provided the pretext for the introduction of the first U.S. ground forces into Vietnam. Anticipating Vietcong attacks against U.S. airbases in retaliation for Rolling Thunder, General Westmoreland in late February urgently requested two Marine landing teams to protect the air base at Danang.... [O]n March 8 , two battalions of Marines..., with tanks and eight-inch howitzers, splashed ashore near Danang where they were welcomed by South Vietnamese officials and by pretty Vietnamese girls passing out leis of flowers. (George C. Herring, America's Longest War: The United States and Vietnam, 1950-1975, 2d ed. (1986), pp.130-131)Several months later, in July, Johnson decided to commit ground forces on a large scale (50,000 immediately, followed by another 50,000 before the end of the year). Johnson did this without going to Congress for authorization; attorney general Nicholas Katzenbach assured Johnson that bypassing Congress was within his prerogatives (Herring, America's Longest War, p.140, citing Katzenbach to Johnson, June 10, 1965, Johnson Papers, National Security File, Country File: Vietnam, Box 17).
One of the reasons Johnson decided to take this approach was that "he feared that going to Congress for authority to wage war in Vietnam would destroy his dream of creating the Great Society at home" (Herring, p.140). He also declined to mobilize the reserves, call up the National Guard, seek a tax increase or do much of anything else to indicate the country was preparing to wage a war (ibid.). While this might have avoided political problems in the short term, in the long run it helped paved the way for disillusion with the U.S. war in Vietnam, especially as it became clear that the conflict was not going to be short.
The leading explanation in the literature for the Vietnam escalation decisions of 1965 used to be, and perhaps still is, that the general commitment to containment of Communism and the specific commitment to not let a Communist regime take power in Vietnam dictated the decisions. However, there were different escalation options on the table and containment doesn't explain why particular ones were chosen and others were rejected. As Y. F. Khong argued in Analogies at War (1992), the Korean War experience and the fear of provoking Chinese intervention weighed heavily on LBJ, inclining him to choose "graduated" escalation options. One consequence of that choice was to make it very likely that the U.S. would not be able to prevail against an adversary willing to pay almost unlimited costs. [Clarification added later: I'm not sure, on re-reading, that this makes sense. I guess what I meant to say was that, while no strategy was likely to succeed in attaining its goals, the 'gradual' options chosen were especially unlikely to succeed. I'm not completely sure that's right, but it seems right.] As early as June 1964, North Vietnamese premier Pham Van Dong had told Canadian diplomat J. Blair Seaborn that "the NLF [Viet Cong] and its supporters were prepared to endure regardless of the cost" (Herring, p.119). That remark proved to be accurate.
Note: Vietnam War is a new index label; previous posts here about the Vietnam War can be found under the label Vietnam in the topics index.
Added later: Re anniversaries, March 9 was the 70th anniversary of the U.S. firebombing of Tokyo; see here. (I may have something more to say about the linked piece later.)
(Post edited slightly after initial posting.)