In 1968, the single bloodiest year of the Vietnam War, half a million American troops were fighting in Southeast Asia. In 2024, Americans are not fighting and dying in Gaza, Ukraine, or any other foreign war. The hundreds of thousands of protesters against the conflict in Gaza remain smaller in numbers and political power than the anti-war demonstrators of the late ’60s; this spring’s campus upheavals were less disruptive. Vietnam tore the Democratic Party apart in 1968. When Senator Eugene McCarthy ran against President Lyndon B. Johnson in the New Hampshire primary in March, the president couldn’t muster 50 percent of the vote, leading Kennedy to jump into the race, Johnson to withdraw, and Vice President Hubert Humphrey to take his place. This year’s political crisis was not a stalemated war but a declining brain. Old age was President Joe Biden’s Vietnam, the June debate his Tet Offensive. Once again, the credibility gap was fatal.