On January 30, 1968, the day of "Tet" (the Vietnamese New Year), the Viet Cong and the North Vietnamese Army launched a major military campaign that would spell the end of Johnson's presidency. The Johnson administration had, up to that point, guaranteed the American public that the United States was on the verge of winning the war (which Johnson had escalated into a full-blown American operation in 1965). The so-called "Tet Offensive" proved that opposition in Vietnam remained much stronger than Johnson had indicated. Video of Viet Cong soldiers storming the American embassy in South Vietnam made the nightly news. On February 27, news anchor Walter Cronkite delivered a still-famous monologue on air in which he declared that while the United States did not look like it would lose the war, it was now obvious that it could not win it either. The anti-war protest movement, which had existed for years, exploded into the mass movement which we remember today. Protesters followed LBJ around on his campaign trail, chanting "Hey! Hey! LBJ! How many kids did you kill today?", among other popular forms of protest. Some historians have described LBJ as being "trapped" in the White House at times, unable to come outside to face the protesters.

In the first Democratic primary, held in New Hampshire on March 12, Johnson won--but only by a narrow margin--against the long-time anti-war Democrat Eugene McCarthy. With this near loss and the chaos of the anti-war movement continuing to escalate around him, Johnson determined that he could not win the general election that year. Nevertheless, Johnson remained president and he and he maintained a series of foreign-policy goals that he hoped to accomplish before the end of his term. He therefore announced on March 31 that he would neither seek nor accept the Democratic nomination for the presidency, and he thereafter focused on ending the war in Vietnam and continued to pursue an arms reduction treaty with the Soviet Union (work that he could do outside the eye of the public, mind you).

Here the story of the Democratic primaries and Lyndon Johnson's final push diverge. In short, Robert Kennedy quickly overtook McCarthy after entering the race, but he was assassinated before the convention, and the far less charismatic Hubert Humphrey ended up taking the nomination at the infamously protested Democratic National Convention of 1968. Johnson, on the other hand, made great strides towards realizing a detente with the Soviet Union, but he had to cancel an arms-reduction summit with Soviet Premier Alexi Kosygin after the Soviet Union invaded Czechoslovakia (this details of this story are an further stain on his legacy, I have to add). In the end, the Democrats ended up with a candidate who could not win and Johnson left office without having achieved a single foreign-policy accomplishment that could compare to his enormous failure in Vietnam.