The rushed retreat from Afghanistan is an ongoing human tragedy, the collision of historical realities with exceptionally poor decisions.

But is this really going to derail President Biden’s entire agenda? Probably not.

Democrats and all the leftist Cassandras on cable TV really need to calm down, because history and common sense suggest that Afghanistan isn’t going to define the Biden presidency — and probably not even the next election.

Democratic Washington is in the grip of panic. The general feeling is that Biden was seen as competent and steady until his Afghanistan withdrawal blew up, with 13 service members now dead and more violence likely to come. (This on top of a resurgence of covid-19, mostly in states where vaccination rates are lagging.)

As Biden’s once-soaring approval ratings plummet, Democrats fear massive losses in next year’s midterm elections and a hobbled presidency. Some are urging that Biden start firing senior members of his Cabinet.

Maybe everybody should just take a deep breath and read a little history.

All presidents get pounded by unforeseen crises — especially overseas, and especially early in their terms when they’re dealing with policies inherited from their predecessors.

This kind of crisis may hasten the end of a presidential “honeymoon,” which has always been ephemeral and these days is pretty much nonexistent. But you’d be hard pressed to name an instance when it marked the turning point in a president’s tenure.

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John F. Kennedy rued his embarrassing failure in the Bay of Pigs. Ronald Reagan reeled from the deaths of more than 200 Marines in Beirut in 1983. (Unlike Biden, Reagan bore responsibility for deploying those troops.)

Bill Clinton saw his presidency rocked in its first year, when 18 U.S. service members were killed in Somalia, having been sent there in the last weeks of George H.W. Bush’s term.

We remember all these disasters, which dominated politics at the time, but none of them can be said to have defined a presidency or even come close.

In terms of foreign policy, Kennedy is chiefly remembered for the Cuban missile crisis, Reagan for the end of the Cold War, Clinton for his foray into Kosovo. Kennedy is remembered as a great president; the latter two won reelection easily.

Moreover, Democrats should recognize that they were always going to lose the coming midterms. I could be wrong — maybe they were poised to buck the trend before Afghanistan went south. Maybe the Nationals were coming back from 10 games out to win the World Series before they sold off their team.

But the fact is that the president’s party almost never gains seats in a midterm cycle. The two presidents before Biden came into office with majorities and promptly lost them, largely because independent voters grew disenchanted.

So it’s practically a given, with the narrowness of their margins and the added challenge of redistricting, that Democrats were going to suffer serious losses next year with or without Afghanistan.

Midterm elections are pretty much never driven by foreign policy, anyway. Will voters a year from now focus on the economy? Yes. The state of the pandemic? Sure. Government spending? Entirely possible.

Bungling the end of a 20-year war in Afghanistan — a withdrawal that most people supported? Very, very unlikely.

Finally, there’s this: Whatever else may be coming unglued right now, the presidency, seven months after Donald Trump’s departure, is no longer a carnival. Which, after all, is the main reason Biden was elected in the first place.

Americans didn’t give Biden the most votes of any president in history — and the largest margin of victory since 2008 — because they thought he’d never make a mistake, or that he’d seamlessly carry out his predecessor’s policy of ending overseas wars.

They voted for Biden because he promised a return to reliable and accountable leadership, a dignified presidency that stayed in the background of American life rather than dominating the culture with toxic lies and misdirection. We’d all had an enough of the unscripted spectacle.

So here he is, at the nadir of his young presidency, speaking solemnly to the public, mostly accepting responsibility for what’s gone wrong, leveling with voters about the peril that still exists and the limits on what he can promise.

It’s possible that in the end, even in crisis, Biden will end up reinforcing for voters the very qualities that made him seem like a desirable alternative in the first place. At a minimum, he will have closed the door on a protracted war in which most Americans — Democrat, Republican and independent — had long ago lost faith.

Will that be enough to save Biden’s party from inevitable setbacks or to guarantee him another term, should he want it? Of course not.

But it doesn’t spell the end of his presidency, either.