Mr. Trump shouldn’t repeat President Biden’s mistakes. An agreement that involves zeroing out U.S. military assistance smacks of Mr. Biden’s willful blindness in withdrawing from Afghanistan. In the middle of the withdrawal in July 2021, Mr. Biden dismissed questions about whether the Afghan government would fall: “Look, we were in that war for 20 years. Twenty years. . . . I want to talk about happy things, man.” This was despite U.S. intelligence repeatedly warning him that the Afghan National Security Forces would struggle to hold the Taliban at bay.

The Afghan government wasn’t ready then, and Europe isn’t ready now, especially given Russia’s increasing might. A report released last month by the U.S. Office of the Director of National Intelligence said that “Moscow’s massive investments in its defense sector will render the Russian military a continued threat.” Gen. Christopher Cavoli, Supreme Allied Commander Europe, warned Congress on April 3 that Russia’s economy is on a “war footing” and that Moscow enjoys strong military support from China, Iran and North Korea. Past European free-riding makes it tempting for the Trump administration to wash its hands of Ukraine, but Mr. Trump must reject this urge.

Like the Taliban, Russia has violated many internationally brokered cease-fires and agreements. Chechnya offers a particularly clear example of Vladimir Putin’s treachery and ambition. After the breakaway republic humbled Russia’s military in 1996—and the two sides signed a “forever” peace agreement—Moscow spent three years rebuilding before forcefully bringing Chechnya back into its domain. Today, Mr. Putin’s territorial designs in Ukraine are clear. He demands the surrender of land across Donetsk, Luhansk, Zaporizhzhia and Kherson, including areas that Russia hasn’t conquered and doesn’t occupy.

Mr. Trump insists that Mr. Putin respects him so much that he won’t reinvade Ukraine after an agreement is signed. Even if he’s correct, Mr. Putin will likely outlast Mr. Trump in office. If Mr. Trump strikes a deal with Mr. Putin and Ukraine’s military readiness and morale plunge without U.S. support, Mr. Putin will be tempted to resume hostilities. Mr. Trump’s 2024 victory was in part a rebuke of the Biden administration’s foreign-policy weakness. A Russian reinvasion would contradict Mr. Trump’s promise to create “peace through strength.”

Finally, Mr. Trump has proved adept at using leverage in negotiating matters from real estate to international relations. But American leverage over Russia is rooted in U.S. military support to Ukraine. The U.S. can’t achieve better relations with Russia unless Mr. Putin needs something from Mr. Trump. Unilaterally ending assistance to Ukraine would eliminate a key American lever to influence Mr. Putin and blunt his territorial ambitions.

The U.S. doesn’t have to back Ukraine indefinitely. Mr. Trump’s insistence that Europe do more is already succeeding. The U.K. has pledged to increase defense spending to 2.5% of gross domestic product by April 2027, and Germany has announced major investments as well. French President Emmanuel Macron has pushed for European countries to devote 3% to 3.5% of their GDP to defense.

These spending increases will take time to bear fruit. But a recent report from the Kiel Institute for the World Economy noted that Europe can make early progress in areas where it already produces supplies that are vital to Ukraine, such as howitzers, tanks, drones and infantry fighting vehicles. This would allow the U.S. to scale down its provision of these materials.

That said, Europe’s defense industrial base shortcomings mean it may take longer for the Continent to produce certain types of ammunition and rocket artillery. Restricting U.S. military assistance in these categories before Europe has time to develop this capacity would give Russia a window to surge forward.

Mr. Trump rightly describes the Afghanistan debacle as one of the most ignominious moments in U.S. history. Mr. Biden never recovered from it: His approval ratings tanked, and it left a stain on his presidency. Mr. Trump is also correct that in many ways the Russia-Ukraine conflict is “Biden’s war.” But if he hastily cuts off assistance to Ukraine, and Russia advances westward, Mr. Trump will go down in history for making the same reckless mistakes as his predecessor.

Mr. Allen served as special assistant to the president and senior director for counterproliferation strategy (2007-09) and majority staff director of the House Intelligence Committee (2011-13). He is author of “Blinking Red: Crisis and Compromise in American Intelligence After 9/11.”