Exclusive: Vindman to retire from military. His lawyer blames White House 'campaign of bullying, intimidation and retaliation' - CNN

"These are choices that no one in the United States should confront, especially one who has dedicated his life to serving it," he added, noting that Vindman "did what the law compelled him to do; and for that he was bullied by the President and his proxies."

Top Pentagon leaders, including Secretary of Defense Mark Esper, have insisted that Vindman is not being targeted for political reasons, but a source familiar with his decision said military officials have communicated to Vindman that the White House has sought to become involved in the promotion process.

In response, Vindman was told that that there have been discussions within the Department of Defense about sending his name forward on a "list of one" or holding his name back until after the election to avoid impacting the promotions of other service members, the source said.

It is "absurd and frightening" for the White House to be involved in promotions at this level, the source added.

Ultimately, Vindman decided to retire from the military rather than attending the National War College, which was his next planned assignment, after speaking with senior Army officials who made clear that there were forces working against his advancement within the military.

Specifically, Vindman was told by senior Army officials that he would no longer be deployable in his area of expertise, which includes Ukraine, the source familiar with the situation told CNN.

He was also told by senior officers he would need a "rehabilitative assignment" even if he had opted to attend the National War College, an option he had been considering before Wednesday's announcement, the source added.

In one case, a senior officer quipped about sending him to "man a radar station in Alaska," the source said.

The White House and Pentagon did not immediately respond to CNN's request for comment.

Impeachment retaliation

Vindman delivered explosive testimony during public impeachment hearings that Trump's push for Ukraine to investigate former Vice President Joe Biden was "inappropriate" and that he knew "without hesitation" that he had to report it.

Vindman said that he reported his concerns out of a "sense of duty," and he defended his fellow witnesses from what he described as "reprehensible" attacks.

Testifying in his Army uniform as an active-duty soldier, Vindman invoked his father's decision to leave the Soviet Union and come to the US, noting that the testimony he was giving would likely get him killed in Russia.