Only two Democrats (and no Republicans) voted against the 1964 Gulf of Tonkin resolution that Lyndon Johnson used as his license to send up to 550,000 U.S. troops to Vietnam. But by 1968, opposition to that war was welling up, primarily but not entirely within the Democratic Party.
LBJ was opposed by antiwar Eugene McCarthy and dropped out of the race. In 1972, Democrats nominated the dovish George McGovern. For nearly half a century, they have been the party less supportive of military intervention.
Not that Republicans have invariably supported it. Ronald Reagan aided the Nicaraguan Contras and intervened in Grenada but withdrew from Lebanon. He built up the military but didn't find much occasion to use it.
George H.W. Bush got approval from the United Nations before asking Congress to authorize the Gulf War. George W. Bush sought U.N. approval for Iraq, too.
Democrats remained obsessed with Vietnam. Their speeches opposing Contra aid and the Gulf and Iraq wars were full of arguments more relevant to the Gulf of Tonkin resolution than to the issue at hand.
Some Democrats disagreed. Bill Clinton used force (without U.N. approval) in Serbia and Kosovo. Almost all Democrats supported intervention in Afghanistan after 9/11.
But almost all congressional Democrats tried to stop George W. Bush's successful surge strategy in Iraq. Hillary Clinton found cause to question the veracity of Gen. David Petraeus.
The surge came too late to salvage the reputation of the Iraq War. Polls now show majorities think it was a mistake. Most Republican politicians seem disinclined to suggest we should intervene anywhere else.
World problems loom: North Korea, Iran, Syria, North Africa. Barack Obama may choose to respond militarily. He has just beefed up missile defense in response to North Korea.
If he follows up on his threat to attack Iran's nuclear program, we could have a 2016 presidential race in which Republican Rand Paul criticizes military action and Democrat Hillary Clinton defends it.
That would be a political turnabout as stark as in the 1960s. Could it happen?