Schlesinger’s point then, and Maher’s now, is that the enemies of liberals do not reside only on the right. In the 1930s and 1940s, some liberals grew so focused on the struggles against fascism and racism—struggles in which communists proved staunch allies—that they refused to acknowledge Joseph Stalin’s crimes. Today, some liberals are so focused on the struggle against American militarism and Islamophobia that they can’t muster much outrage against ISIS. According to Schlesinger, occupying the Vital Center means opposing totalitarianism wherever you find it, regardless of whether it claims the mantle of progressivism, as the Soviet Union did during his time, or anti-imperialism, as jihadists do now.

So far, so good. Where Maher goes wrong is in forgetting two other lessons of the liberal anti-totalitarian tradition. The first is to be precise about what you’re opposing. The second, to not get so carried away with your own virtue that you end up justifying terrible crimes.

Let’s start with the point about precision. At their best, the liberals of the early Cold War trained their fire on Stalin, a particular ruler in a particular country at a particular moment in time. When they began making sweeping generalizations about communism per se—forgetting that communist regimes and movements varied depending on their time and place—they got in trouble. In the 1940s and 1950s, the myth of a monolithic communist movement blinded some liberals to the fact that European socialists and communists—many of whom had their own national loyalties—could prove effective allies against Soviet power. “A recalcitrant Communist Yugoslavia,” in George Kennan’s words, “proved more resistant to Soviet Communist pressures than any non-Communist regime would have been likely to do.”

Even more tragically, in the 1960s, some Cold War liberals could not distinguish between the communism of Joseph Stalin and the communism of Ho Chi Minh. By seeing Ho only as a communist, they overlooked the fact that many Vietnamese saw him primarily as an anti-colonialist. And by pretending that the ideological distinctions between North and South Vietnam resembled the ideological distinctions between East and West Germany, pro-war liberals ignored the reality that in Indochina, America’s allies were no more democratic than its communist foes.

“The tragedy of Vietnam,” wrote Schlesinger in 1967, “is the tragedy of the catastrophic overextension and misapplication of valid principles.” And the engine of that overextension and misapplication was ignorance. Most pro-war liberals simply didn’t know enough about Vietnam to realize that their anti-communism was leading them astray. As Graham Greene wrote of Alden Pyle, the idealistic CIA agent in his novel, The Quiet American, “He was impregnably armoured by his good intentions and his ignorance.”