Russian President Vladimir Putin’s implied threat to turn the Ukraine war into a broader nuclear conflict presents President Joe Biden with choices rarely contemplated in the atomic age, including whether to raise the alert level of U.S. nuclear forces.

This turn of events is all the more remarkable for the fact that less than a year ago, Putin and Biden issued a statement at their Geneva summit that seemed more in keeping with the idea that the threat of nuclear war was a Cold War relic. “Nuclear war cannot be won and must never be fought,” they agreed.

Putin on Sunday told his top defense and military officials to put nuclear forces in a “special regime of combat duty,” but it was not immediately clear how that might have changed the status of Russian nuclear forces, if at all. Russia, like the United States, keeps its land-based intercontinental ballistic missiles, or ICBMs, on a high state of readiness at all times, and it is believed that Russian submarine-based nuclear missiles, like America’s, are similarly postured.

Putin indicated he was responding to economic sanctions imposed by the United States and other Western nations in recent days for his invasion of Ukraine, as well as “aggressive statements regarding our country,” which he did not further explain.

The Biden administration was assessing Putin’s move, which it said unnecessarily escalates an already dangerous conflict. In fact, Putin’s words amount to the kind of threat rarely heard even during the Cold War period, when vastly larger nuclear arsenals of the United States and the former Soviet Union threatened the world with nuclear Armageddon.

U.S. officials, while disturbed by Putin’s words, indicated they did not know what he intends. But it is so rare for an American or Russian leader to issue an implied nuclear threat, particularly in the current context of the war in Ukraine, that the risk of it going nuclear cannot be dismissed. In Russia, like in the United States, the president has sole authority to order a nuclear strike.

The United States and Russia have the two largest nuclear arsenals in the world, by far. They include weapons that can be delivered by aircraft, submarine and land-based ballistic missiles. The only time in history that nuclear weapons have been used in combat was when the United States twice bombed Japan in August 1945, and at that point the U.S. had a global monopoly on nuclear weapons. The Soviet Union successfully tested its first bomb in 1949.

Daryl Kimball, executive director of the Arms Control Association, said Putin’s order to put his nuclear forces on higher alert was regrettable but not a complete surprise given his previous implied threats against any nation that tried to stop him in Ukraine.

“Inserting nuclear weapons into the Ukraine war equation at this point is extremely dangerous, and the United States, President Biden, and NATO must act with extreme restraint” and not respond in kind, Kimball said. “This is a very dangerous moment in this crisis, and we need to urge our leaders to walk back from the nuclear brink.”

According to U.S. nuclear doctrine, the weapons’ alert level is central to their role in deterring attack. The idea is that being prepared to respond on short notice makes an enemy less likely to attack in the first place and risk retaliation that would do incalculable damage.

A counterargument is that having ICBMs, which the Pentagon calls the most responsive portion of its nuclear arsenal, on high alert during a crisis compresses a president’s decision-making room and leaves open the possibility of ordering them launched in response to a false alarm. The 400 deployed U.S. ICBMs are armed at all times.

Some arms control experts have argued for taking ICBMs off high alert by separating the missiles from their nuclear warheads. But in a crisis, perhaps like the one implied by Putin’s alert order Sunday, a decision to re-arm the missiles would be taken as an escalatory move that could make the crisis even worse.

During the Cold War, U.S. and Russian weapons were not only more numerous but also in a higher state of readiness. President George H.W. Bush in 1991 took the historic step of ordering U.S. nuclear-capable strategic bombers off alert as part of a broader move to reverse the nuclear arms race. The bombers have remained off alert ever since.

There is no evidence that the Biden administration has reciprocated in any sense to Putin’s announcement that he was ordering his nuclear forces in a “special regime of combat duty” — perhaps in part because it was unclear what that means in practical terms.

Nor was there word from Washington of evidence that Putin had taken worrying steps such as loading nuclear weapons on all or a portion of Russia’s nuclear-capable air fleet or sending additional ballistic missile submarines to sea.

In addition to his strategic nuclear force, Putin has at least a couple thousand so-called nonstrategic nuclear weapons, such as shorter-range ballistic and cruise missiles. They are called nonstrategic because they cannot reach U.S. territory. But that is little comfort for the countries in Europe that are within range of those weapons. The United States has about 200 nonstrategic weapons in Europe; they are bombs that would be delivered by Europe-based aircraft.

For years, some U.S. officials have worried that Putin, if faced with the prospect of losing a war in Europe, might resort to the use of nonstrategic nuclear weapons, thinking it would quickly bring the conflict to an end on his terms.