European Union Ambassador Gordon Sondland testified Wednesday in one of the week's most-anticipated impeachment hearings.

Driving the news: In his opening statement, Sondland said that he worked with Rudy Giuliani "at the express direction" of President Trump on matters involving Ukraine.

He also confirmed that a quid pro quo preconditioning a White House meeting in exchange for an investigation into the Biden family's business dealings in Ukraine took place and "reflected President Trump's desires and requirements."

He said that he believed that the resumption of military aid to the country came to be conditioned on a Biden-linked investigation as well.

Other highlights:

Sondland testified that other senior officials, including Secretary of State Mike Pompeo and acting White House chief of staff Mick Mulvaney, knew about the quid pro quo for the Zelensky White House meeting.

Under questioning from House Intelligence Commitee Chairman Adam Schiff (D-Calif.), Sondland said Trump only wanted Ukrainian President Volodymyr Zelensky to publicly announce the Biden-linked investigations: "He didn't actually have to do them, as I understood it."

Sondland said that he never heard directly from Trump that the military aid was conditioned on an announcement of investigations, saying that assumption was his "own personal guess." He later added, "By the 8th of September, it was abundantly clear to everyone that there was a link."

Sondland refused to say whether he believed Trump's assertion in a Sept. 9 phone call that there was no quid pro quo involving the military aid. While he texted Bill Taylor, the top U.S. diplomat in Ukraine, that there was no quid pro quo after the call, he testified: "I was just trying to convey what [the president] said on the phone."

The other side: Trump told reporters outside the White House during Sondland's testimony that he "doesn't know" him, adding: "This is not a man I know well. He seems like a nice guy though."

It's the latest walk-back from the president about his relationship with Sondland, donated $1 million to Trump's inaugural committee. He similarly has claimed not to know a number of senior officials caught up in the impeachment inquiry.

Our thought bubble, via Axios' Jonathan Swan: Unless former national security adviser John Bolton does a last minute reversal and voluntarily testifies — a scenario that appears implausible — Sondland will be the witness who serves as the test case of the White House’s theory of impeachment.

The theory goes that nothing —literally nothing — that happens in these hearings could convince 20 Republican senators to convict Trump.

What to know: Sondland is a key figure in alleged efforts by Trump and Giuliani to pressure Ukraine to investigate Joe Biden and the 2016 presidential election.

He revised his closed-door testimony to reflect that he told an aide to Zelensky that military assistance would not be released until Zelensky issued a statement agreeing to investigate Burisma, a gas company with ties to Biden's son.

State Department official David Holmes said in his closed-door testimony that he overheard a call between Trump and Sondland about "the investigation" on July 26 — one day after the Trump-Zelensky call at the heart of the impeachment inquiry.

Watch:

Read:

The past hearings:

Go deeper: