On July 25 of last year, then Major Alexander Vindman listened, with other officials, to a conversation between Trump and the newly elected Ukrainian president, Volodymyr Zelensky. Vindman as the National Security Council’s director for European affairs had organized the call.

“I would like you to do us a favor,” Trump told Zelensky, working his way to the subject of Joe Biden: “There’s a lot of talk about Biden’s son, that Biden stopped the prosecution, and a lot of people want to find out about that, so whatever you can do with the attorney general would be great. Biden went around bragging that he stopped the prosecution, so if you can look into it …”

“Having seen something ‘questionable (in the call),’ Vindman properly notified his superiors,” former White House Chief of Staff John Kelly said. “When subpoenaed by Congress in the House impeachment hearings, Vindman complied and told the truth. He did exactly what we teach them to do from the cradle to the grave,” he said. “He went and told his boss what he just heard.”

Vindman made an official complaint to John Eisenberg, the chief NSC lawyer, and Michael Ellis, a White House lawyer and Eisenberg’s deputy. Vindman’s reporting set in motion all that was to come. A whistleblower shared details of the call, and Vindman later realized that he could be compelled to testify.

On Monday night, Vindman had an interview with NBC’s Lester Holt. In the interview, Vindman said that he did not know for sure who filed the whistleblower complaint. He stated that he suspected the complaint to be an impeachable offense, but his only duty was to report what he had heard. The Senate later acquitted President Trump, and the White House stated that there was nothing wrong with the call.

Vindman said he was untroubled by the consequences of his reporting. “I had to choose between the president and the Constitution. I was aware of the fact that I could be compelled to testify. But I chose the Constitution. No Army officer wants to be put in that position, but there I was.”

Lieutenant Colonel Alexander Vindman: From zero to hero and back again

Read Next: Lieutenant Colonel Alexander Vindman: From zero to hero and back again

According to the Atlantic, John Kelly said that Vindman, a key witness in President Donald Trump’s impeachment inquiry, was right to raise concerns about Trump’s July call to Ukraine’s president.

Kelly also believes that Vindman, who was fired from the White House, told the truth during testimony before House investigators last fall.

Vindman has since left the military after more than 20 years of service. He suspected that if he were to have stayed in the Army he would “have ended up at a radar station in Alaska.” President Trump called him a “Never-Trumper,” a characterization that Vindman rejects. Rather, Vindman says that when he took the position at the White House he was not, but now he is one.

“Coming forward ruined my career, I still believe doing what’s right matters.” He told Lester Holt that coming forward and defending the Constitution may have been one of his proudest moments.

Vindman is currently pursuing his doctorate with Johns Hopkins University School of Advanced International Studies (SAIS).