Russian President Vladimir Putin said Tuesday that Moscow is “suspending” its participation in New START, the only remaining nuclear arms control treaty between the United States and Russia.

“Our relations have degraded, and that’s completely and utterly the U.S.’s fault,” Putin said during his annual state of the nation address. He added: “If the U.S. conducts tests, then so will we. Nobody should have any illusions that global strategic parity can be destroyed.”

The announcement sparked alarm among Western officials, who warned about the disintegration of the global arms-control architecture.

Here’s what to know about the New START accord.

Q: Why do Russia and the United States have an arms-control treaty?

A: New START is the latest in a series of agreements between the United States and Russia — or its predecessor, the Soviet Union — to reduce the size of their respective nuclear arsenals.

The two superpowers raced to build their nuclear weapons stockpiles in the early decades of the Cold War in a risky competition that raised fears of a mutually destructive nuclear war. In the late 1960s, President Lyndon B. Johnson called for talks with Moscow to limit each side’s strategic arms. Formal Strategic Arms Limitation Talks (SALT) began under Johnson’s successor, President Richard M. Nixon, in Finland in 1969. In 1972, Nixon and Soviet General Secretary Leonid Brezhnev signed a treaty on Anti-Ballistic Missile defense systems — meant to shoot down incoming missiles — and an interim agreement placing restrictions on the construction of intercontinental ballistic missile (ICBM) silos.

Under subsequent agreements, including START I and SORT, both countries pared down their nuclear arsenals substantially. But arms control negotiations were often rocky, and other proposed treaties never entered into force. In 2002, the George W. Bush administration unilaterally withdrew from the Anti-Ballistic Missile Treaty on grounds that it curtailed the United States’ ability to protect itself from “terrorists” and “rogue states.”

Q: What’s in the treaty?

A: The New START agreement was signed by then-President Barack Obama and then-Russian president Dmitry Medvedev in 2010. Beginning in early 2011, the United States and Russia had seven years to scale back their stockpiles of strategic offensive arms — broadly, nuclear warheads deployed by missiles, planes or submarines that can travel long distances.

Specifically, each country is limited to 700 deployed intercontinental ballistic missiles, submarine-launched ballistic missiles and heavy bombers equipped to transport nuclear weapons; 1,550 nuclear warheads on these vehicles; and 800 “deployed and non-deployed” launchers.

These include caps on Russian long-range nuclear weapons that can reach the United States, according to the State Department.

Under the terms, U.S. and Russian inspection teams are also supposed to be able to conduct 18 short-notice inspections of the other country’s nuclear sites per year, to verify that the other side is holding up its end of the deal.

The agreement was criticized initially for being unambitious, said John Erath, senior policy director for the Center for Arms Control and Non-Proliferation.

The deal allows both Russia and the United States to hold onto hundreds of powerful nuclear weapons that, if deployed, could wreak widespread death and destruction. The quantity is sufficient to act as a deterrent for launching a nuclear weapon, Erath said.

The treaty “has done its job in that there has not been a renewal of an arms race between the U.S. and Russia,” he added.

Q: Have the United States and Russia complied with it?

A: Both countries met the limits spelled out in the treaty by February 2018 and appear to have remained at or below them since then, according to the State Department.

The future of the agreement has appeared in jeopardy at several points since it was signed, however. President Donald Trump’s administration called the treaty “deeply flawed” since it didn’t encompass shorter-range, “tactical” nuclear weapons. Talks to extend the agreement stalled under Trump, and his administration withdrew from a separate agreement with Russia that banned intermediate-range missiles.

The Biden administration reached an agreement with Moscow in early 2021 to extend New START until February 2026 “because it was clearly in the security interests of our country and actually in the security interests of Russia,” Secretary of State Antony Blinken told reporters in Greece on Tuesday.

But regular inspections mandated by the agreement have not been held for the past three years — initially because of the coronavirus pandemic, and later because relations soured after Russia invaded Ukraine. In November, Russia unilaterally postponed a technical meeting with U.S. officials about the treaty, citing “political reasons.”

In a report to Congress last month, the State Department said the United States “cannot certify the Russian Federation to be in compliance” with the treaty since Russia had refused to allow U.S. inspections on Russian territory. The report raised concerns about Russia’s compliance with the warhead limit, which the latest data indicated the country had nearly reached.

Q: What could Putin’s announcement mean?

A: It doesn’t mean Russia is withdrawing from the treaty entirely, Putin said. But Moscow will not allow NATO countries to inspect its nuclear arsenal, he said, accusing the alliance of helping Ukraine conduct drone strikes on Russian air bases that host strategic bombers, part of the country’s nuclear forces.

“It’s entirely symbolic,” Erath said, since Russia had not been permitting inspections anyway. The move appears to be aimed at pressuring President Biden and allies to approach Moscow about ending the war in Ukraine, “so Russia can dictate the terms under which that would happen.”

Still, Putin’s announcement has raised alarm among Western officials about the deterioration of nuclear disarmament efforts, at a time when tensions between the United States and Russia have reached a recent peak after Russia’s invasion of Ukraine last year.

NATO Secretary General Jens Stoltenberg warned that “the whole arms control architecture has been dismantled,” adding, “I strongly encourage Russia to reconsider its decision.”

Overstating the impact of the announcement carries risks, Erath said — particularly if the United States makes concessions that might signal to other countries that nuclear weapons are an effective form of diplomatic leverage.

But Russia can’t afford an arms race, either. Mired in a costly conventional war in Ukraine, “the last thing they need is to try to get into a nuclear weapons-building competition with the U.S.,” Erath added.

Blinken on Tuesday called Putin’s decision “deeply unfortunate and irresponsible.”

“We’ll be watching carefully to see what Russia actually does,” he said. “We’ll, of course, make sure that in any event we are postured appropriately for the security of our own country and that of our allies.”

The Washington Post’s John Hudson in Athens, Emily Rauhala in Brussels and Mary Ilyushina and Robyn Dixon in Riga, Latvia, contributed to this report.