Lt. Col. Alexander Vindman, who served as a key witness in President Trump's impeachment trial, announced Wednesday that he has moved to retire from the military after 21 years of service amid fears that he will "forever be limited" due to political backlash over his testimony.

The big picture: The president fired Vindman in February as the leading Ukraine expert on the National Security Council for being "insubordinate," but top military leaders including Secretary of Defense Mark Esper claim Vindman had not been politically targeted.

• In his testimony, Vindman called Trump's push for Ukraine to investigate former Vice President Joe Biden "inappropriate" and said he reported his concerns on the matter due to a "sense of duty."
• Sen. Tammy Duckworth (D-Ill.) vowed last week to block the promotions of 1,123 senior military officers until Esper confirmed that Vindman's promotion would go through.

What they're saying: "Through a campaign of bullying, intimidation and retaliation, the president of the United States attempted to force LTC Vindman to choose: Between adhering to the law or pleasing a president. Between honoring his oath or protecting his career. Between protecting his promotion or the promotion of his fellow soldiers, " Vindman's lawyer, Amb. David Pressman, said in a statement.

• "These are choices that no one in the United States should confront, especially one who has dedicated his life to serving it," Pressman added.