President Donald Trump ordered government agencies on Monday to propose revoking two regulations for each new one they issue, fulfilling a promise he made shortly after his election.

The military and regulations related to national security will be exempt from the executive action, administration officials familiar with the action said. They requested anonymity to discuss the action before Trump had signed it.

“It’s going to be hard to implement, just because changing rules involves going through detailed administrative processes and soliciting public comment,” said Darrell West, director of governance studies at the Brookings Institution in Washington. “So it’s not a situation where an agency head can come in and kill a regulation overnight.”

The process generally takes months, and can be challenged with a lawsuit by aggrieved parties, West said. “Particularly in the environmental area, you can’t just make a change,” West said. “You have to demonstrate that the costs outweigh the benefits if you want to kill something, so that involves looking at health risks and the economic impact of regulations.”

To carry out the president’s intent, agencies would need to write a regulation that kills another rule, said Jeff Holmstead, a partner at Bracewell LLP who is a former assistant administrator at the Environmental Protection Agency.

“I assume over time there will be — they’ll figure out an implementation strategy that makes some sense of these,” Holmstead said.

Agencies will propose regulations to revoke when they write new ones, the administration officials said, and the White House will approve the revocations.