“We easily forget how fascism works,” writes the Yale historian Timothy Snyder, author of the distinguished Bloodlands: Europe Between Hitler and Stalin, this week in “The Battle in Ukraine Means Everything: Fascism Returns to the Country It Once Destroyed,” an impassioned, eloquent essay in The New Republic.

We also easily forget how intellectual observers work in reporting to us on complex developments abroad that few of us could hope to parse without their help. Snyder means not only to instruct and enlighten readers of that magazine and theFrankfurter Allgemeine Zeitung, where a version of the essay appeared. He wants to awaken the West to oppose and indeed to defeat Vladimir Putin’s efforts to lock Ukraine into Russia’s sphere of influence (or even its direct sovereignty, as Putin has already done in Crimea), as well as into its anti-Western “Eurasian” political and economic imperium - Russia’s answer to what Putin considers a decadent European Union.

A note below Snyder’s essay announces that he and his New Republic editor Leon Wieseltier have “planned a congress of international and Ukrainian intellectuals to meet May 16 to 19 in Kiev under the heading Ukraine: Thinking Together.” So his essay is meant to rouse Europe and the United States to stop Putin in a confrontation that for Snyder is as fateful as that of fascism and Communism in the Spanish Civil War in 1937 or — as he argues explicitly — of Stalin’s pact with Hitler in 1939, when both were preparing to crush Poland and extend their respective empires.

Snyder notes rightly that Stalin’s anti-fascist rhetoric was as opportunistic as his signing of the 1939 pact with the fascist Hitler. But Stalin did fight German and Italian fascism in the Spanish proxy war, even as he was also crushing the democratic left within the anti-fascist ranks there, as Orwell, another great intellectual witness, reported in Homage to Catalonia when few on the left wanted to hear it.

So the historical precedents are ambiguous. Surely Putin’s Russia is more intimately opportunistic and brutal toward Ukraine than Stalin’s Russia was toward Spain. Snyder insists that Putin’s motives are as imperialist and even fascist as Stalin’s. He notes that Putin is even courting right-wing nationalist politicians in western Europe who prefer his bold aggression, at least from a distance, to what they see as Brussels’ militarily weak, culturally dissolute, bureaucratic suppression of their own nationalist yearnings.

Since few of us can second-guess reports grounded in scholarship and experience as exhaustive as Snyder’s, we’re as beholden to him in trying to understand Ukraine as we often are to other knowledgeable witnesses elsewhere. Sometimes they’re fatefully wrong, as were Paul Berman and Fouad Ajami in claiming to reveal truths about Islamicism and Iraq that necessitated armed intervention. Sometimes they’re protecting their own intellectual turf and predilections as vigorously as they are the peoples they’re claiming to champion.

But, just as often, intellectual witnesses are indispensably right, as were Jonathan Schell in Viet Nam and Timothy Garton Ash in Eastern Europe in 1989. Is Snyder one of them? His account is invaluable in peeling back layers of duplicity surrounding others’ claims about who are the fascists and who are the democrats in Ukraine and about whether being a Russian speaker necessarily means being a Russia supporter. But an historian who is trying to make history may become a little apocalyptic and wishful, and while Snyder is surely right to remind us that most Ukrainian nationalists aren’t fascists, as Putin claims, that doesn’t make them as democratic and freedom-loving as Snyder keeps telling us....