Lt. Col. Alexander Vindman, a national security aide who testified against President Donald Trump during the impeachment inquiry, announced his retirement from the Army on Wednesday in a scathing statement that accused the president of running a “campaign of bullying, intimidation and retaliation.”

In a statement, Vindman’s attorney David Pressman said that his client resigned after over 21 years of military service because his career in the armed forces “will forever be limited” due to political retaliation. Pressman charged that Trump and his allies were to blame.

After the lieutenant colonel was featured as a critical witness in the House impeachment inquiry, Trump vowed revenge. Trump, along with his staffers and personal associates, started a purge of witnesses from the White House in February.

In February, Trump fired Vindman as the top Ukraine expert on the National Security Council and also dismissed his twin brother Lt. Col. Yevgeny Vindman, who also worked in the White House.

“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,” Pressman said.

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

Top Pentagon officials, including Secretary of Defense Mark Esper, have said that Trump and his allies are not targeting the lieutenant colonel for political reasons.