A treaty isn’t really a treaty if one side has violated it for a decade. That’s why the United States is right to demand changes to Russian behavior or it will pull out of the Intermediate-Range Nuclear Forces Treaty.

The major turning point came Monday in Brussels when the U.S., with the full backing of its NATO allies, declared that Moscow was in “material breach” of the 1987 agreement by deploying a new ground-based cruise missile with a banned operational range of between 500 and 5,500 kilometers, sufficient to threaten Western Europe.

Such weapons, as well as ballistic missiles with similar ranges, were banned by the treaty, and those then in operational status were destroyed in 1988 by the United States and the Soviet Union, the two signatories to the treaty. The treaty covers both conventional and nuclear weapons

The crisis over the new Russian missile is a clear signal that the time has come to either abandon the treaty or modify and broaden its coverage to take into account major technological changes in weapons over the past three decades, and the proliferation of intermediate range missiles in the arsenals of Iran, China and North Korea.

Although Secretary of State Mike Pompeo announced at the NATO foreign ministers’ meeting that the United States will “suspend our obligations” under the pact in 60 days from Dec. 4, he left the door open if “Russia returns to full and verifiable compliance.” Russia should do what it can to preserve the treaty.

Russian President Vladimir Putin’s blustery response to the U.S. announcement of the 60-day deadline seemed to indicate his country was ready for a new arms race. “Many other countries — about a dozen of them there already, probably — produce such weapons, while Russia and the United States have limited themselves bilaterally. Now, apparently, our American partners believe that the situation has changed so much that the United States should also have such weapons. What is the answer from our side? Yes, simple: Then we will do it too.”

Given the huge complications involved in drafting a new treaty or modifying the INF document to not only include new weapons technologies but to invite participation by China, North Korea and Iran, the simplest course would be to walk away from the treaty. But that course entails great risks. For example, what happens if Russia restores its arsenal of nuclear weapons targeted on Europe? Will Europe refuse to reciprocate and deny forward bases for U.S. weapons designed to restore a military balance?

The danger of a new Europe-centered arms race makes the possibility of drafting a new agreement worth pursuing. And Russia has signaled that it is interested. After President Trump announced Oct. 20 that he was considering withdrawing from the INF treaty he sent National Security Adviser John Bolton to Moscow to explore Russian interest in a new negotiation. The Russian government has said the INF was on a list of topics Mr. Putin planned to discuss with Mr. Trump in Buenos Aires last week during the G-20 summit.

Separately, Russia has indicated over the past decade or more that it considers U.S. drones, such as the Predator, equivalent to the weapon systems banned by the INF, and has charged that the ABM launchers deployed by the United States in Eastern Europe could be used to launch Tomahawk cruise missiles banned by the INF. More broadly, it has shown an interest in negotiating the status of hypersonic weapons that because of their unique flight paths are not covered by the INF, and in arranging an increase in the number of nations subscribing to limits on such weapons.

But Russia must choose. Does it want negotiations to head off an arms race it really cannot afford, or does it want to continue to behave in threatening ways? There must be a visible change in Russian behavior before the ground is ready for bold new arms control agreements, however necessary they appear to be.