(Reuters)

President-elect Donald Trump on Thursday called for the United States to expand its nuclear arsenal, after Russian President Vladi­mir Putin said his country’s nuclear potential needs fortifying, raising the specter of a new arms race that would reverse decades of efforts to reduce the number and size of the two countries’ nuclear weapons.

In a tweet that offered no details, Trump said, “The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.”

Trump’s position represents a radical shift in thinking. Russia and the United States have worked for decades at first limiting, and then reducing, the number and strength of nuclear arms they produced and maintained under a Cold War strategy of deterrence known as “mutually assured destruction.” Republican and Democratic presidents have pursued a policy of nuclear arms reduction.

Trump’s tweet came shortly after Putin, during a defense ministry meeting, talked tough on Russia’s stockpile of nuclear weapons. Putin said that Russia is the strongest nation in the world but that it cannot rest.

“We need to strengthen the military potential of strategic nuclear forces, especially with missile complexes that can reliably penetrate any existing and prospective missile defense systems,” Putin said in an apparent reference to a planned NATO troop buildup in Eastern Europe.

The Trump camp offered only slightly more explanation of the president-elect’s comment later in the day, when communications director Jason Miller said in a statement that Trump “was referring to the threat of nuclear proliferation and the critical need to prevent it — particularly to and among terrorist organizations and unstable and rogue regimes.”

Miller added that Trump believes in “the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength.”

Trump’s tweet was in keeping with earlier comments he has made. During an October debate, he criticized this country for lagging behind Russia in its nuclear program. “We are old, we’re tired, we’re exhausted in terms of nuclear,” he said. “A very bad thing.”

He also suggested that South Korea and Japan develop nuclear weapons to protect themselves from the threat posed by North Korea.

The United States has just under 5,000 warheads in its active arsenal and more than 1,550 deployed strategic warheads, a number that fluctuates, according to Daryl G. Kimball, executive director of the Arms Control Association. In an October assessment by the State Department Bureau of Arms Control, Verification and Compliance, Russia has about 400 more nuclear warheads than the United States does. But the United States has about 170 more delivery systems than Russia.

(Peter Stevenson/The Washington Post)

Under the New START Treaty, the main strategic arms treaty in place, the United States and Russia must deploy no more than 1,550 strategic weapons by February 2018. Kimball said both countries appear to be on track to meet that limit, which will remain in force until 2021, when they could decide to extend the agreement for another five years.

Since President George H.W. Bush’s administration, it has been U.S. policy not to build new nuclear warheads. Under President Obama, the policy has been not to pursue warheads with new military capabilities.

The U.S. military is in the beginning stages of updating its nuclear triad, which covers the delivery systems — bombers, submarines and intercontinental ballistic missiles. Last year, the Pentagon estimated that it must spend an average of $18 billion a year over 15 years, starting in 2021, to replace weapons that already have been refurbished and upgraded beyond their original shelf lives.

But independent experts have estimated that the cost of modernizing the aging nuclear arsenal could reach $1 trillion over 30 years, according to the Arms Control Association.

“If Donald Trump is concerned about the rising costs of the F-35, he will be shocked by the skyrocketing costs of the current plan to modernize the U.S. nuclear arsenal,” Kimball said. “Trump and his people need to explain the basis of his cryptic tweet. What does he mean by expand, and at what cost?”

But others say that nuclear weapons and the principle of deterrence are essential components of national security and that the Obama administration’s efforts to further reduce its nuclear weapons have been wishful thinking.

Michaela Dodge, a Heritage Foundation policy analyst specializing in nuclear weapons and missile defense policy, said that the White House in its 2010 Nuclear Posture Review made the erroneous assessment that there was little likelihood of conflict with Russia. Yet Moscow is in the midst of a large-scale nuclear weapons modernization program and has violated many arms control treaties that it signed, she said.

“There is already an ongoing nuclear arms race, except now the United States isn’t racing,” she said in a telephone interview. “It’s mostly Russia and China.”

Dodge has called for the incoming Trump administration to spend more on its nuclear weapons program. She also said that the United States should withdraw from nonproliferation treaties that have not worked and consider resuming nuclear test explosions, the last of which was conducted in 1992.

“Nuclear weapons present an existential threat to the United States,” she said. “So to continue to have strong deterrence is a national priority.”

Robert Jervis, a national security policy professor at Columbia University, said the remarks by Putin and Trump do not necessarily mean a new arms race is on the horizon.

“Not yet, but we’re seeing the sorts of dynamics that could lead to one,” he said. “But we’re umpteen steps away from that.”