A senior official fired from the White House after testifying against Donald Trump during impeachment hearings has quit the military.
Lt Col Alexander Vindman was the victim of a "campaign of bullying, intimidation and retaliation" by the president, his lawyer said.
Lt Col Vindman's "concern" over a phone call between Mr Trump and Ukraine's president led to a Senate trial.
The Republican-majority Senate acquitted the president in February.
What has Lt Col Vindman's lawyer said?
A statement said he had retired after 21 years of military service because his future would be "forever limited".
The statement from his representative, David Pressman, said Lt Col Vindman had spoken publicly only once on the matter – under subpoena from Congress.
He said: "LTC Vindman did what the law compelled him to do; and for that he was bullied by the president and his proxies."
Mr Pressman added that he had "put the interests of his country ahead of his own. "LTC Vindman's patriotism has cost him his career," he said.
Lt Col Vindman tweeted confirming his decision:
On Monday, Senator and Iraq War veteran Tammy Duckworth said she had still not been assured by Secretary of Defence Mark Esper that the White House would not interfere with Lt Col Vindman's promotion to full colonel.
There has been no response from the White House yet to Lt Col Vindman's retirement.
What led to his White House sacking?
Testifying in Congress last November, Lt Col Vindman, a Ukraine expert, said he was "concerned" after hearing Mr Trump's "improper" phone call on 25 July 2019 with Ukrainian President Volodymyr Zelensky.
The call led to Mr Trump's impeachment in December by the Democrat-majority House of Representatives for abuse of power and obstruction of Congress. Democratic lawmakers argued that the president had dangled US aid in exchange for political favours.