A Democratic senator and veteran is demanding an explanation from President Donald Trump's defense secretary of the "disgraceful situation" that saw a key impeachment witness retire from the military in response to what his lawyer described as presidential "bullying," and she will block over 1,000 senior military promotions until she gets it.

Lt. Col. Alexander Vindman requested retirement from the military Wednesday in response to a White House "campaign of bullying, intimidation, and retaliation" led by the president, his lawyer said in a statement first reported by CNN.


Vindman, an Iraq War veteran and Purple Heart recipient who served on the National Security Council as a Ukraine expert, testified against Trump in House impeachment hearings, characterizing some of his actions as "improper."

Trump was impeached by the House but acquitted by the Senate earlier this year, and in the aftermath, the president swiftly fired Vindman before moving on to target other senior US government officials considered disloyal.

Vindman, who has served in the armed forces for more than two decades, remained in the military after he was removed from the NSC, and Pentagon leaders said he would not be subject to retaliation.

But in recent weeks, questions have been raised about his future in the military and his expected promotion to colonel.

Army Secretary Ryan McCarthy and Secretary of Defense Mark Esper approved Vindman's promotion after a Pentagon inspector general inquiry looking at Vindman and allegations of "inappropriate behavior"— conducted at the request of the White House — did not find any reason to block his promotion, Politico reported Wednesday.

Reuters reported that the recommended promotion had not yet been sent to the White House when Vindman abruptly decided to retire.

"Lt. Col. Vindman's decision to retire puts the spotlight on Secretary of Defense Mark Esper's failure to protect a decorated combat veteran against a vindictive Commander in Chief," Sen. Tammy Duckworth, a US Army veteran who lost her legs because of injuries she sustained during the Iraq War, said in a statement Wednesday afternoon.

She said: "Secretary Esper's failure to protect his troops sets a new, dark precedent that any Commander in Chief can interfere with routine merit-based military promotions to carry out personal vendettas and retaliation against military officers who follow duly-authorized subpoenas while upholding their oath of office and core principles of service."

Last Thursday, the Illinois senator tried to shield Vindman's promotion from retaliation by blocking 1,123 senior military promotions until she received a written assurance from Esper saying that he would not block Vindman's promotion to colonel, which she said she still has not received.

The senator said in statement Wednesday that she would continue to put a hold on these promotions until Esper provides a "transparent accounting" of what her office described as a "disgraceful situation."

While Vindman confirmed that he was retiring from the military, he has not personally explained the reasons for his departure. His lawyer, however, said Vindman "did what the law compelled him to do; and for that he was bullied by the President and his proxies."

He added: "Vindman's patriotism has cost him his career."

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.