On Tuesday, Lt. Col. Alexander Vindman is testifying in the impeachment inquiry about what he heard on President Trump’s July phone call with Ukraine’s newly elected president and why it troubled him.

Who he is

Vindman is the top White House expert on Ukraine, a staffer on the National Security Council and a decorated war veteran. He was born in Ukraine when it was still the Soviet Union and immigrated to America as a child with his twin brother, who is also a national security official. As children, the Vindman twins were briefly featured in a Ken Burns documentary about the American immigrant story. Vindman enlisted in the U.S. Army and earned a Purple Heart in Iraq.

Why he matters

He is one of the first witnesses testifying publicly who listened in on Trump’s July phone call with Ukrainian President Volodymyr Zelensky. The fact that he is testifying undercuts Republicans’ argument that Democrats have no firsthand witnesses to alleged wrongdoing by Trump. Vindman is one of several national security officials who listened to the call and were disturbed by it. Another, Jennifer Williams, will testify alongside him.

He also provides evidence that the Trump White House offered Ukraine a quid pro quo after overhearing a diplomat tell Ukrainian officials that they would get an Oval Office meeting in exchange for investigations.

What we learned from his private testimony

A couple of things. He was very concerned about how Trump conducted the call, and it’s possible that his decision to raise those concerns led to the White House trying to cover up the call. After he heard Trump ask Ukraine’s president to investigate former vice president Joe Biden and his son Hunter, Vindman said, he relayed his concerns to a lawyer on the National Security Council, who instructed him to stay quiet about it. Vindman also said the rough transcript that the White House released in September omits Biden-specific references in the conversation.

Vindman also said he heard Gordon Sondland, U.S. ambassador to the European Union, tell Ukrainian officials who were visiting the White House that for their president to get an Oval Office meeting with Trump, they needed to investigate Democrats. Vindman testified that he told Sondland “it was inappropriate and we were not going to get involved in any investigations.”

Key quote from his private testimony

“There was no doubt."

That was Vindman’s answer when asked whether he had any doubt that Trump was asking Ukraine’s president to investigate his political rivals on that phone call in order for Ukraine to get what it wanted. That, too, undercuts Republicans’ defense that Trump was merely focused on corruption when he brought up the Bidens — and offers a potential motive for Trump withholding military aid to Ukraine before he talked to Ukraine’s president in July.

What he doesn’t say in his private testimony

What Trump was thinking. Vindman is a staffer on the National Security Council in the White House, but he’s not a political aide who was consulting with Trump.

What to watch for in his public testimony

Will Vindman make a convincing case that Trump’s call with Ukraine’s leader wasn’t “perfect,” as the president claims? He cuts a striking figure as a witness: a White House staffer and an immigrant who has dedicated his life to serving the United States. Also, will he be subject to ugly attacks about his loyalty to America, as he was by some Trump allies after he testified privately? There are signs he will. On the eve of his testimony, Sen. Ron Johson (R-Wis.) questioned whether he was an anti-Trump bureaucrat.