Our cause in Ukraine is inspiring. It probably won’t stay that way.

At this point, Putin’s unprovoked war has provided the United States — and the world — with a simple narrative and uncontroversial choices. Russia, the behemoth, is isolated from the world and reeling from resistance. Plucky Ukraine stands its ground and pleads for help amid mounting casualties.

But this period of easy choices and simple story lines is coming to an end. We’re reaching the phase of the crisis where the interests of Ukraine and the United States are no longer perfectly aligned, and the balance will be hard to navigate.

Like any capable leader in his situation, Ukrainian President Volodymyr Zelensky wants to repel the invaders and keep his country whole. His best (and probably only) hope for driving the Russians out is to draw NATO deeper into the war.

And so Zelensky pleads movingly with the West for leftover Russian-made fighter jets and a no-fly zone over Ukraine. If he’s going to hold out indefinitely, he needs the United States and Europe to do more than sanction Russia and ferry weapons across the border.

The United States, however, has a different bottom line, as it did throughout the 40-year Cold War: to avoid a shooting war with a country that keeps nuclear missiles aimed at U.S. cities.

“We will not fight a war against Russia in Ukraine,” Biden said flatly Friday. “Direct confrontation between NATO and Russia is World War III.”

As Biden has said, if Russia even tiptoes over the line into NATO territory, we’ll have little choice but to take up arms directly. The same might conceivably be true if Putin unleashes chemical weapons — an option for which he appears to be laying a false pretext.

But absent that kind of broader menace, Biden will be forced to take a realpolitik view that most of us will find hard to stomach. No matter how unjust Ukraine’s fate, he must continue to reject any measure that threatens to put U.S. troops in direct conflict with the Russians.

Anything else would be reckless. Yes, the absorption of Ukraine into Russia would be a human tragedy and geopolitical nightmare. But a shooting war between NATO and Russia would constitute an existential crisis that some large segment of the planet might not survive.

This might mean pressuring Zelensky to accept a negotiated solution that is patently unjust — if it’s even possible. No one wants to say it now, but America would sooner see Ukraine cede some territory than risk all-out war. It would be an imperfect solution, but one in which we’d all live to fight another day.

You could argue that any compromise would perversely reward Putin’s aggression and invite even more, and it’s true: Our own policies, notably the nonresponse to Putin’s annexation of Crimea in 2014 and former president Donald Trump’s public slavishness toward Russia, led directly to where we are now.

But the universe of outcomes in Ukraine is limited now. The most likely scenario involves Putin unleashing savagery on the country to possess it, and it ends with Ukraine leveled, Zelensky dead and Russian troops on the Polish border. You’d have to think a negotiated alternative that leaves Ukraine partly intact, if that window opens, would be preferable.

Such odious calculations aren’t new; we just haven’t had to contend with them for a while. We remember the Cold War now as a principled stand against tyranny — and it was.

But the story of the nuclear age was also one of pursuing moral ends through a cascade of compromises that were morally ambiguous at best.

At the dawn of the Cold War in 1945, both President Franklin D. Roosevelt and President Harry S. Truman felt compelled to reach unsettling terms over the future of Europe with Joseph Stalin. For the United States, the priority was defeating Japan, where America wanted Stalin’s help.

That deal is generally remembered as having sold out Poland and other countries in the Soviet orbit, whose freedoms were quickly crushed under grinding Russian tanks.

For decades afterward, American presidents enabled repressive regimes — Chile, South Africa, Iran and so on — all in the cause of containing Communism. We abided assassinations and undercut democracies when we thought we had to do so.

In retrospect, it’s easy to see these allowances as misguided and shameful. But from the end of World War II to the fall of Communism, East and West did not come to all-out war, and not a single nuclear-armed missile was fired. You might say we avoided Armageddon at the cost of our collective conscience.

It would be nice if Ukraine required no such painful calculations — if Putin would simply put his tanks in reverse and plead for economic mercy. But Biden must know that he needs to prepare the country for a more agonizing endgame. His job, in the best case, will be to make a negotiated outcome palatable abroad and at home.

If he does, count on this: Our much-needed sense of unity will be shattered overnight. Republicans will scream that Biden is the new Neville Chamberlain, while internationalists in the president’s party will complain that he walked away from human rights.

In that event, though, Biden will have checked Russian aggression without letting NATO get drawn into another world war. History tells us that in a showdown between nuclear powers, that’s what leadership is.