President Trump and Russian President Vladimir Putin hold a joint news conference after their meeting in Helsinki on July 16. (Grigory Dukor/Reuters)

PRESIDENT TRUMP is justified to question the status quo with the Intermediate-range Nuclear Forces Treaty, signed in 1987 between the United States and the Soviet Union, which eliminated an entire class of nuclear-armed missiles in Europe with a range between 310 miles and 3,400 miles. Under President Vladimir Putin, Russia has developed and tested a ground-launched cruise missile, designated the 9M729, in violation of the treaty, according to State Department reports. An impasse over the issue has existed since 2013. Russia denies that it has violated the treaty, but some reports say the new cruise missile is already deployed.

What’s the right response? “So we’re going to terminate the agreement, and we’re going to pull out,” Mr. Trump declared. Theoretically, if his threat leads to a new negotiation and verifiable fix, he will have accomplished something. Another possibility, more worrisome, is that once the United States abandons the treaty, Mr. Putin will feel unconstrained about building more such weapons. This year, in a speech to the Federal Assembly, he showed off videos of futuristic systems such as a nuclear-powered cruise missile that could fly for thousands of miles. Never mind that this fantasy has yet to work; Mr. Putin was taunting the West for underestimating Russian power. If the United States abandons the treaty, Mr. Putin might claim the moral high ground, too.

The INF Treaty came about after NATO agreed in the late 1970s to deploy ground-based cruise and ballistic missiles in Europe to counter Soviet missiles. The “dual track” NATO decision meant deploy and negotiate. The pressure was successful, over time and despite protests. Soviet leader Mikhail Gorbachev gave up the missiles, joining President Ronald Reagan in a verifiable treaty. Both sides stood down.

Mr. Trump seems determined to repeat the cycle of coercion. “We are going to develop the weapons,” he said. “If . . . they say, ‘Let’s not develop these horrible nuclear weapons,’ I would be extremely happy with that. But as long as somebody is violating that agreement, then we’re not going to be the only one to adhere to it.”

National security adviser John Bolton correctly observed that the U.S.-Russia treaty does not cover China. He said on a visit to Mr. Putin in Moscow, “This is a Cold War bilateral ballistic-missile-related treaty — in a multipolar ballistic-missile world.” Does Mr. Trump intend to seek a new agreement that would encompass Asia? We doubt it. Meanwhile, a new arms race would strain the U.S. budget, test the patience of European allies and pose fresh hurdles to the extension of the 2010 New START strategic arms treaty, which deserves to be prolonged.

Supposedly, at the Helsinki summit, Mr. Trump developed a rapport with Mr. Putin. Now’s the time to use it. Instead of walking away, the president should attempt to fix the INF Treaty’s flaws, verifiably, and with Mr. Putin, perhaps at their next summit. That would be smarter than repeating the last dangerous cycle of the Cold War.