Fearing that Donald Trump will pull out of NATO if he’s reelected president, and alarmed that the U.S. Congress is close to cutting off military aid to Ukraine, many Europeans are discussing the possibility of building their own independent defense force to deter and, if necessary, stave off an expansionary Russia.

Here’s the problem: The task is impossible. At least for the next several years, Europe requires the North Atlantic Treaty Organization—an alliance that must be led by the United States—for its protection.

NATO’s 30 European members (in addition to their trans-Atlantic partners, the U.S. and Canada) have been spending more money on defense since 2014, when Russian President Vladimir Putin annexed Crimea, and even more since 2022, when he invaded all of Ukraine. Although just three of the countries were fulfilling their pledge to spend at least 2 percent of their GDP on defense, 18 are on track to hit or surpass the mark this year.

But measured in dollars or euros, that isn’t nearly enough without America’s contribution. Of the $1.3 trillion that the 32 NATO nations combined spend on defense, more than half—$860 billion—is spent by the United States. True, the U.S. military has global missions, so much of that money is earmarked for defenses in Asia, the Middle East, South America, and so forth. But our global military is also a network; ships, planes, troops, and so forth, which might normally be deployed in one region, can be re-deployed to a different region. The entire budget could theoretically benefit Europe. But let’s say just half of the budget is devoted primarily to........

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