Lt. Col. Alexander Vindman, ‘98, who testified against President Donald Trump during his impeachment inquiry, announced on Wednesday that he will retire from the Army, citing bullying and intimidation from Trump.

On Wednesday, the 45-year-old Army lieutenant colonel ended his 21 years of service to the Army. David Pressman, Vindman’s lawyer, released a statement on why the lieutenant colonel chose to retire.

“Through a campaign of bullying, intimidation and retaliation, the president of the United States attempted to force … Vindman to choose: Between adhering to the law or pleasing a president,” Pressman wrote. “Between honoring his oath or protecting his career. Between protecting his promotion or the promotion of his fellow soldiers. These are choices that no one in the United States should confront, especially one who has dedicated his life to serving it.”

Last November, Vindman, top Ukraine expert on the National Security Council (NSC), became a star witness after he raised his concerns about Trump calling Ukraine President Volodymyr Zelensky to investigate Joe Biden, former vice president and current democratic presidential candidate, and his son, Hunter Biden. Vindman said Trump’s call was “improper” and that it undermined U.S. national security. Vindman’s testimony prompted the House of Representatives to impeach Trump in December for abuse of power.

In Februrary, Vindman, as well as his brother, Lt. Col. Yevgeny Vindman, were fired by Trump from an assignment of the NSC. This move was characterized by many as retaliation from the president. Trump tweeted about the firing on Feb. 8.

“Fake News [CNN] & MSDNC keep talking about ‘Lt. Col.’ Vindman as though I should think only how wonderful he was,” Trump wrote. “Actually, I don’t know him, never spoke to him, or met him (I don’t believe!) but, he was very insubordinate, reported contents of my ‘perfect’ calls incorrectly and was given a horrendous report by his superior, the man he reported to, who publicly stated that Vindman had problems with judgement, adhering to the chain of command and leaking information. In other words, ‘OUT.’”

An abnormal delay in Vindman’s promotion to colonel led many government officials to grow more concerns over the effect of Vindman’s testimony.

Last week, Sen. Tammy Duckworth promised the Pentagon and Defense Secretary Mark Esper to delay more than 1,000 military promotions if Lt. Col. Vindman’s promotion was interfered with. After the Lt. Col. announced his retirement, Duckworth released a statement on Wednesday.

“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,” Duckworth wrote.

In a Twitter post on Wednesday, Vindman confirmed his decision to retire after Pressman’s statement was released.

“Today I officially requested retirement from the U.S. Army, an organization I love,” Vindman wrote. “My family and I look forward to the next chapter of our lives.”

After graduating from Binghamton University with a bachelor’s degree in history in 1998, the lieutenant colonel enlisted in the military. Vindman is a recipient of a Purple Heart medal for the wounds he suffered during the Iraq War in 2004 from an improvised explosive device (IED) attack.

“LTC Vindman’s patriotism has cost him his career,” Pressman wrote. “Today our country loses a devoted soldier, but it is incumbent upon all of us to ensure it does not lose the values he represents.”