The American Left has always been pro-war
The modern left has always been for interventionism and war, not the least because war guarantees the State more centralized power, excuses wartime socialism (redistributionism), permits unique measures to stifle dissent, and enables unelected deep-state bureaucrats to keep their actions secret.
There was never a switch. What there was, in the United States anyway, was a protracted period of gaslighting.
Once you recognize the truth of this, it resolves the left's seeming inconsistency and its present salivation over the prospect of war with Russia (a historical irony to be sure).
First, their ostensible antipathy for war was actually an intentional means to aid and abet a foreign adversary, and then it became a go-to for caltropping domestic rivals.
Where the latter, their domestic strategy, is concerned, contrast the left's vociferous critiques of the Iraq War (which was supported by the Dems in Congress), with its relative silence concerning the Libyan War, the Syrian War, the flattening of Yemen, and Obama's various illegal military actions in the Near East.*
The left developed a reputation for being anti-war in the 20th Century primarily because they protested the Vietnam War (i.e. protecting Vietnamese Christians from Communist butchers), military build-up and readiness against the Soviet Union, and against the State Department's / CIA's proxy wars against Communists in the Near East and in Latin America.
They weren't anti-war. Rather, they were anti-Western, anti-capitalist, anti-American, and anti-Christian. Above all, they were against America winning wars.
They wanted an atheistic, materialistic, internationalist communism to succeed. Moreover, they wanted it to take root and flourish in America.
Look at the so-called Kent State Massacre. They weren't anti-war. They were pro-Year-Zero. The student protesters who Neil Young and others eulogized were literal Maoists and Cultural Revolutionaries, protesting an intervention to stop genocide in Cambodia. They were protesting in support of Pol Pot and his Year Zero, which resulted in the murder of millions of innocents.
*Though we have a number of names for the warmongers who have or had situated themselves on the right, like Romney, McCain, Powell, Graham and others—neocons, neoliberals, etc.—they are no more rightists than was Woodrow Wilson. They may be less ideological, and indifferent to the leftist long game, but they kick with the same reflex as the Leninist.)