Trump's unexpected nuclear weapons pledge has a couple possible catalysts

Expert: Why does Trump keep tweeting?

Donald Trump unexpectedly tweeted about "greatly" strengthening and expanding America's nuclear capability Thursday, the first time he mentioned the weapons on the platform in more than a month.

Trump tweet: 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 transition team did not immediately respond to a request for comment on why Trump brought up nuclear weapons. But two recent events may have moved the issue to the front of his mind.

The president-elect on Wednesday met with the chief executives of Boeing and Lockheed Martin, companies bidding on the contract to replace the Minuteman nuclear arsenal. He also had a separate meeting with top military leaders.

Separately, Russian President Vladimir Putin gave a speech Thursday in which he called for a modernization of Moscow's nuclear program.

"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, according to a translation from AFP.

Trump spokesman Jason Miller said in a statement to NBC News that "President-elect 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. He has also emphasized the need to improve and modernize our deterrent capability as a vital way to pursue peace through strength."

Miller did not respond to a follow-up question asking whether that meant Trump was not, in fact, calling for more nuclear weapons.

Trump: US must expand nuclear capability

Trump last brought up nuclear weapons on Twitter in mid-November, shortly after his unexpected electoral win. He attacked The New York Times, claiming that the newspaper wrongly reported that he said "more countries should acquire nuclear weapons."

The newspaper, though, only said that Trump has "suggested" arming other countries to boost U.S. security, according to PolitiFact. He told The New York Times in March that the U.S. could potentially be better off if Japan and South Korea had nuclear weapons due to their proximity to North Korea.

In the same interview Trump said that the "biggest problem, to me, in the world, is nuclear, and proliferation."

UPDATED: This story was updated to include a statement from Trump spokesman Jason Miller.