Updating these systems will also be costly. The Congressional Budget Office estimates it will cost $400 billion to update the nuclear triad over the next 10 years.

The modernization also entails refurbishing nuclear warheads and improving the aging facilities where the work is done. While the work on warheads has been ongoing for decades, it accelerated under Obama, Reif said.

The Pentagon is also updating the command and control systems that facilitate communication with nuclear forces. Additionally, former Defense Secretary Chuck Hagel set into motion a program to overhaul management within the U.S. nuclear force to address professional and ethical lapses.

The Department of Defense and Energy Department did not immediately respond to inquiries into whether they had taken any steps as a result of any executive action by Trump.

The White House did not immediately respond to a request for clarification on Trump’s tweet.

The tweet also contained a factual error. Trump’s first action as president was not ordering the Defense Department to conduct a Nuclear Posture Review.

That executive order followed several other orders and memos, including those related to repealing Obamacare, approving controversial oil pipelines, expediting environmental reviews for infrastructure projects, hiring more immigration and customs officers, and securing the border with Mexico.