By David Sanger and Edward Wong, New York Times–

The United States on Friday terminated a major treaty of the Cold War, the Intermediate Nuclear Forces agreement, and it is already planning to start testing a new class of missiles later this summer.

But the new missiles are unlikely to be deployed to counter the treaty’s other nuclear power, Russia, which the United States has said for years was in violation of the accord. Instead, the first deployments are likely to be intended to counter China, which has amassed an imposing missile arsenal and is now seen as a much more formidable long-term strategic rival than Russia.

The moves by Washington have elicited concern that the United States may be on the precipice of a new arms race, especially because the one major remaining arms control treaty with Russia, a far larger one called New START, appears on life support, unlikely to be renewed when it expires in less than two years.

At a moment when the potential for nuclear confrontations with North Korea and Iran is rising, the American decision to abandon the 32-year-old treaty has prompted new worries in Europe and Asia, and warnings that echo an era that once seemed banished to the history books. The resurgence of nuclear geopolitics was evident in the Democratic debate on Tuesday night, when presidential hopefuls grappled with whether the United States should renounce “first use” of nuclear weapons in any future conflict.

“The United States and Russia are now in a state of strategic instability,” Ernest J. Moniz, the former energy secretary, and Sam Nunn, the former Georgia senator who helped draft the legislation that funded the drastic reduction in former Soviet nuclear forces, write in a coming article in Foreign Affairs ominously titled “The Return to Doomsday.” “Not since the 1962 Cuban missile crisis has the risk of a U.S.-Russian confrontation involving the use of nuclear weapons been as high as it is today. Yet unlike during the Cold War, both sides seem willfully blind to the peril.”

Others are less concerned about the implications with Russia, noting that the treaty is limited, covering only a narrow class of missiles.

President Barack Obama considered terminating the treaty when Moscow was first accused of violating its terms. On Thursday, just as his aides were confirming the American withdrawal and blaming Russia for the breakdown, President Trump told reporters that Russia “would like to do something on a nuclear treaty” and added later, “So would I.” But he appeared to be discussing a broader treaty that would involve China — which has said it has no intention of negotiating a limit on its arsenal.

In fact, the administration has argued that China is one reason Mr. Trump decided to exit the I.N.F. treaty. Most experts now assess that China has the most advanced conventional missile arsenal in the world, based throughout the mainland. When the treaty went into effect in 1987, China’s missile fleet was judged so rudimentary that it was not even a consideration.

Today hundreds of missiles in southeast China are within range of Taiwan, the self-governing democratic island supported by the United States. Missiles at other sites can hit Japan and India, and there are Chinese missiles that can strike the United States territory of Guam and other potential targets in what American strategists call the second-island chain.

“Unilateral constraint was a losing proposition: China developed the world’s foremost force of missiles precisely within the ranges that I.N.F. would prohibit,” said Andrew Erickson, a professor of strategy at the United States Naval War College. “So this increasingly antiquated treaty had no future.”

Until now, the Trump administration has held off on testing new missiles that would violate the treaty; under its terms, even testing is prohibited. But that stricture lifts on Friday, and the first test of new American intermediate-range missiles is likely to begin within weeks, according to American officials familiar with the Pentagon’s plans.

The first, perhaps as early as this month, is expected to be a test of a version of a common, sea-launched cruise missile, the Tomahawk. It would be modified to be fired from the ground. (The treaty prohibited intermediate-range ground-launched missiles, but not missiles launched from ships or airplanes.) If successful, officials say, the first ground-launched cruise missiles could be deployed within 18 months or so — if the United States can find a country willing to house them.

That would be followed by a test of a new mobile, ground-launched ballistic missile with a range of 1,800 to 2,500 miles, before the end of the year. But that would be an entirely new missile, and it is not likely to be deployed for another five years or so — meaning the very end of the Trump presidency, if he is re-elected.

But the question is where to deploy them. “I don’t think the Europeans want to host them,” Gary Samore, the director of the Crown Center for Middle East Studies at Brandeis University and the chief nuclear strategist at the National Security Council under Mr. Obama, said on Thursday. In Asia, he noted, the two countries where it would make most sense to deploy the missiles would be Japan and South Korea, though any move to put the missiles there could infuriate China.

“The real question is where and whether or not there would be pushback,” said Bonnie S. Glaser, a senior adviser for Asia at the Center for Strategic and International Studies. “The most obvious place is someplace in Japan.”

Mr. Samore noted that the fate of New START, which governs the strategic weapons the United States and Russia have deployed, “is much more important than I.N.F.” Senior military officials agree, but have added that once the I.N.F. treaty dies, it is hard to imagine a negotiation to renew New START, which expires in February 2021, right after the next presidential inauguration.

Even if it is renewed, Mr. Samore noted that in coming years, the source of strategic instability may not come just from nuclear weapons but also “from space weapons, artificial intelligence and cyber — and there we have no restraints.”

But it is China’s rocket forces that have focused the attention of the Pentagon and the Trump administration. In 2017, Adm. Harry B. Harris Jr., then the head of United States Pacific Command, said in congressional testimony that the People’s Liberation Army Rocket Force controls the “largest and most diverse missile force in the world, with an inventory of more than 2,000 ballistic and cruise missiles.” He pointed out that the United States capability lagged because of its adherence to the treaty with Russia, and that if China were a signatory, 95 percent of its missiles would be in violation.

But deploying a counterforce to Taiwan would be too provocative, officials say, and Japan may have hesitations: Prime Minister Shinzo Abe would have to consider the blow that would result to relations between Beijing and Tokyo, which have been improving.

China’s fury at deployment of American ground-based missiles in an Asian nation probably would be even greater than its reaction in 2016 and 2017 to plans to install an American antimissile system in South Korea.

For more than a year after the announcement of the deployment, Beijing denounced the move and called for a wide boycott of products from South Korea, whose companies then suffered. The Americans began deploying the system, commonly known as THAAD, in March 2017, and Beijing did not relent on its actions against South Korea until that October. Communist Party leaders feared the United States was laying the groundwork for an expansive antimissile system across Asia.

Chinese officials have also balked at any attempt to limit their missiles with a new treaty, arguing that the nuclear arsenals of the United States and Russia are much larger and deadlier.

“The Trump idea of a trilateral arms control agreement is not realistic,” Mr. Samore said. “The Chinese are not going to codify an inferior number of weapons compared to the United States and Russia. And Russia and the U.S. won’t give China equal status.”