With the war on Islamist terrorism as with the pandemic, America can declare victory all it wants. The timing of the end — in both cases — is outside our control.

Twenty years after the attacks on America of September 11, 2001, President Biden’s withdrawal of American troops from Afghanistan was supposed somehow to signal an end to the hostilities.

The August 2021 attack that killed 13 American service members at the Kabul airport was a reminder that an end to hostilities isn’t something that can be declared by our side unilaterally. I mean, it can be declared — Mr. Biden tried to — but it can’t actually be achieved so long as there’s an enemy out there gunning for us.

The pandemic is the same way. Governments and corporate headquarters can set all the “back to the office” dates they want. But with the delta variant breaking through to infect even vaccinated individuals, return dates are being delayed, just as mask mandates and testing sites are returning.

Mr. Biden announced that vaccinated people could socialize indoors mask-free, and he stopped masking at his own appearances. Then the president’s mask returned, and the guidance changed. As in the case of war, declaring the pandemic over doesn’t make it so.

The clash between militant Islam and America didn’t start on September 11, 2001. It reaches back decades earlier — to the seizure of the American embassy in Tehran in 1979, the attack on the Marine Barracks in Beirut in 1983, the Pan Am 103 bombing in 1988, the first World Trade Center bombing in 1993, the bombings of the American embassies in Kenya and Tanzania in 1998, the bombing of the USS Cole in 2000.

Broadly writ, the violent conflict between the Christian West and armed Islam stretches back even earlier, to the clash at the Gates of Vienna in 1683 and to the Spanish Reconquista of 1492. It’s wishful thinking to imagine that it will end precisely 20 years after the September 11, 2001 attacks, or that an American withdrawal from Afghanistan will bring the war to an end.

Likewise, even if Covid eventually abates, new viruses will threaten. Communist China, whose laboratories might have released this disease and whose secretiveness worsened its initial spread, will persist as a challenge to American economic and national security interests.

Does that mean America should simply give up on trying to influence events — stop trying to win, either against Covid or China or militant Islam? No. Defeatism is as erroneous as are premature, and unwarranted, declarations of victory.

Foreign policy pundits, mentioning Korea, Vietnam, Iraq, and Afghanistan, point out that it’s been a long time since America has decisively won a war. They are forgetting the Cold War. Some historians might claim it as a Soviet implosion, but I see it as an American victory.

American pressure, in the form of moral clarity and a military buildup, forced the Soviet collapse (with some help from John Paul II, Lech Walesa, Andrei Sakharov, Natan Sharansky, and Lane Kirkland). When Reagan and his advisers like Richard Pipes, Albert and Roberta Wohlstetter, Jeane Kirkpatrick, and Richard Perle said the Cold War was winnable, people laughed at them, or worse, and accused them of being lunatics, dangerously steering America and the world toward nuclear annihilation.

Victory in the Cold War resulted from American actions, but in another sense it wasn’t really an American choice. A loss — Europe and North America under Moscow’s totalitarian domination — would have been unacceptable. The same is true of the current confrontations with China and militant Islam. They are different from our foes in the Cold War. The conflicts are similar, though, in that we can’t afford to lose.

The timing of an American victory cannot be taken for granted. It and other post-September 11 foreign policy challenges are best met, though, with that Cold War-winning balance of humility and ambitious determination.