If you want peace, prepare for war, was the Roman maxim.

Following that injunction the U.S. maintains its place as first among superpower equals, even if it is no longer the hyperpower that it was in the 1990s after the collapse of the Soviet Union. But with its depleted armed forces unprepared to fight a major war, or perhaps any kind of war, on its own, Europe (Brexit Britain included) has come to resemble the rich and feeble late Roman Empire that depended on unreliable mercenaries for its security.

¡°The foundations of the world order are being shaken to their core,¡± U.K. Defense Secretary Grant Shapps recently warned in a speech. Russia, China, Iran and North Korea are challenging the status quo and America¡¯s armed forces are overstretched. Shapps¡¯ description of a ¡°pre-war¡± crisis is right ¡ª but what is his Conservative government and its European partners prepared to do about it?

Britain devotes just over 2% of gross domestic product to defense, the minimum NATO requirement. Many of its European allies have a worse record. Yet as recently as 1984, five years before the fall of the Berlin Wall, 5.5% of the U.K.¡¯s GDP was devoted to the military.

After the Iowa caucus in the U.S., the odds have shortened on Donald¡¯s Trump return to the Oval Office. In 2020, Trump told Ursula von der Leyen, the European Commission president that ¡°NATO is dead.¡± It would be prudent to take him at his word. If a Trump administration abandons Ukraine in its hour of need and makes good on threats to make Europe take responsibility for its own protection, a revolution in military and diplomatic thinking will be required. The continent looks wholly unprepared for it.

NATO celebrates its 75th birthday this year. The trans-Atlantic alliance has been extraordinarily successful at keeping the peace ¡ª but with or without Trump, its American underpinning can¡¯t be guaranteed in perpetuity. Geography dictates, however, that Russia will always be a factor in European affairs. Talking to diplomats in Asia in recent weeks, I detected a similar pessimism about America¡¯s long-term commitment. Meantime, China isn¡¯t going anywhere; it¡¯s been the dominant political and cultural power in the region for millennia.

Earlier this month , Adm. Bob Bauer, the Dutch chairman of NATO¡¯s military committee, warned Europe to prepare for war with Russia within 20 years ¡ª and that¡¯s the optimistic view. At around the same time, Estonian Prime Minister Kaja Kallas declared that the alliance may have only between three to five years. Ukraine is Europe¡¯s front line of defense; if Kyiv is forced to accept a ceasefire, then the Baltic states are likely to be the Kremlin¡¯s next target,...