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MOSCOW — The national security adviser, John R. Bolton, is in Moscow this week to explain to officials President Trump’s decision to pull out of a 1987 arms-control pact.

Mr. Trump and his hard-line aides, particularly Mr. Bolton, have long expressed their displeasure with the agreement, the Intermediate-range Nuclear Forces Treaty, because they say Russia is in violation of the terms and China is not a signatory.

“Unless Russia comes to us and China comes to us and they all come to us and they say, ‘Let’s all of us get smart and let’s none of us develop those weapons,’ ” America would pull out and start building new nuclear arms, Mr. Trump said after a campaign rally on Saturday.

In response, the Kremlin spokesman, Dmitri S. Peskov, hinted at a new arms race, saying Russia would be forced also to develop new weapons “to restore balance in this sphere.”

The proposal and the Kremlin’s reaction raised immediate questions about an aging arms control treaty that few people under the age of 30 even knew existed. Is it really so important that its demise would touch off a global arms race?

What is the I.N.F. Treaty?

The treaty resolved a crisis of the 1980s when the Soviet Union deployed a missile in Europe called the SS-20, capable of carrying three nuclear warheads. The United States responded with cruise and Pershing II missiles.

By the time President Reagan and Mikhail S. Gorbachev, the Soviet leader at the time, cut the deal to ban the weapons in 1987, the intermediate-range missiles had come to be seen as a hair trigger for nuclear war because of their short flight times — as little as 10 minutes.

This was particularly troubling to the Soviet command, which could be wiped out by a “bolt from the blue” strike before it could order a retaliatory attack. Partly in response to this shortcoming, Moscow developed a “dead hand” trigger to fire its arsenal at the United States without an order from the leadership, based on computers interpreting radiation and seismic sensors.

In 2014, during the Ukraine crisis, a government newspaper published an article saying this system using “artificial intelligence” to order nuclear war was still operative, though not switched on in peacetime.

The treaty prohibited land-based cruise or ballistic missiles with ranges between 311 miles and 3,420 miles. It did not cover air- or sea-launched weapons, such as the American Tomahawk and Russian Kalibr cruise missiles that are fired from ships, submarines or airplanes, and easily fly similar distances.

Is Russia actually cheating?

It certainly seems so. It was the Obama administration that first accused Russia of violating the treaty in 2014 as the crisis in Ukraine ratcheted up tensions. American officials say Moscow is all but openly deploying a prohibited missile that the West calls the SSC-8, a land-based cruise missile that menaces European nations.

Even during the Obama administration, the United States argued that Russia was in violation of the treaty because it had deployed prohibited tactical nuclear weapons designed to intimidate Europe and the nations of the former Soviet states that have aligned with the West.

But the Trump administration’s greatest worry may be in Asia, where the 1980s pact now constrains the United States from placing short- and intermediate-range missiles on land to respond to China’s efforts to carve out a sphere of influence and keep naval forces at bay in the Western Pacific.

The Chinese, though not a signatory to the treaty, weighed in Monday, saying they also opposed the United States’ unilateral withdrawal.

Mr. Bolton, speaking to the Echo of Moscow radio station in Moscow, responded that, “the Chinese are not participants in this agreement and want it preserved.”

That was hardly a surprise, he said, adding, “If I were Chinese, I would say the same thing.”

How has Russia reacted?

Asked about the possibility of a United States withdrawal earlier this month, President Vladimir V. Putin mused about nuclear Armageddon, saying Russians are ready to launch a retaliatory strike because they know they will go to heaven in a nuclear war.

“The aggressors should know: Revenge is inevitable and they will be destroyed,” Mr. Putin said. “And we, as victims of aggression, will go straight to heaven as martyrs while they will just croak.”

Others have been somewhat more restrained.

“Any action in this field will prompt a reaction,” Sergey V. Lavrov, the Russian foreign minister, told journalists before a meeting with Mr. Bolton.

This ranged from official warnings of a new arms race, with an outside expert threatening that Russia would develop a swarm of small drone-borne bombs to hit the United States, to suggestions that the move was a bluff on the eve of the midterm elections in the United States.

On Monday, Mr. Peskov, the Kremlin spokesman, denied that Russia had violated the treaty, saying that “on the contrary” it was the Americans who had breached its spirit. Russia argues that American antimissile batteries in Europe could be used to fire offensive weapons, and that armed United States drones fly within the ranges prohibited by the treaty for cruise missiles.

With Mr. Bolton in town, Rossiskaya Gazeta, the government newspaper, ran an article under the headline “Trump Turns to Blackmail,” suggesting that he was applying the same hardball tactics he has on trade agreements to a nuclear deal.

How do the Europeans see the issue?

Since Russia’s new intermediate range missiles threaten Europe, it was European leaders who most loudly protested withdrawal from the treaty.

Germany’s foreign minister, Heiko Maas, said in a statement that the treaty has “been an important pillar of our European security architecture,” while numerous analysts have noted that the issue has the potential to drive a new wedge between the United States and Europe at a time of deep stresses to trans-Atlantic relations.

Maja Kocijancic, the European Union’s spokeswoman for foreign affairs and security policy, said in a statement that, “the United States and the Russian Federation need to remain engaged in constructive dialogue to preserve the I.N.F. Treaty,” because “the world doesn’t need a new arms race.”

But with Russia preparing to deploy a hypersonic missile that is not covered by existing arms control agreements, with China deploying intermediate range missiles and the United States responding by modifying cruise missiles to deploy in Asia, many experts maintain the world is already in engaged in one.