John Rood, the Pentagon’s top policy adviser, has drafted a resignation letter and plans to resign, a U.S. official tells Fox News.

President Trump wished Rood well in a tweet on Wednesday.

"I would like to thank John Rood for his service to our Country, and wish him well in his future endeavors!" Trump said.

It’s not immediately clear why Rood is resigning or who ordered him to leave, the official said.

“He was told to resign,” the official said.

But Rood was involved in the process of sending military aid to Ukraine – the central issue during the Trump impeachment trial.

In May, Rood sent a letter to lawmakers certifying that Ukraine had taken “substantial actions” to combat corruption. When the letter was leaked to the press in late September, it undercut Trump’s argument that his administration withheld military aid to Ukraine because of widespread corruption in the country.

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A morning press briefing is scheduled for Wednesday with the Pentagon’s top spokesman in the Pentagon briefing room.

Bloomberg News first reported Rood’s resignation.

Fox News' Alex Pappas contributed to this report.