Washington (CNN) President Donald Trump's former national security adviser John Bolton made waves this week when he said he'd testify at Trump's impeachment trial if subpoenaed.

For 17 months, Bolton was Trump's top adviser on national security. He was fired in September, one day before Trump lifted the freeze on US military assistance for Ukraine. Bolton's role in Trump's orbit put him at the center of several important moments throughout the Ukraine affair.

Here's a chronological breakdown of what Bolton could testify about. Most of these details come from other witnesses who testified in the House impeachment inquiry. It's likely there is even more for Bolton to share, including things that aren't yet public, but it's hard to know for sure.

Conference call with the 'three amigos'

According to congressional testimony, Bolton had a June 10, 2019, phone call with the "three amigos," the three US officials who were deputized by Trump to manage his controversial dealings with Ukraine. The "amigos" were then-Energy Secretary Rick Perry, US Ambassador to the European Union Gordon Sondland, and then-US Special Envoy for Ukraine Kurt Volker.

The three were involved in pushing Ukrainian officials to announce an investigation into former Vice President Joe Biden, in exchange for a meeting with Trump or much-needed US military aid. According to Sondland's testimony about the June conference call, they told Bolton about "the entire Ukraine strategy," and Bolton "agreed with the strategy and signed off on it."

It's not clear if they explicitly discussed plans to pressure Ukraine, and the call happened before Trump froze the military assistance. Bolton's testimony could shed new light on these internal conversations and reveal which Trump officials knew about the Ukraine pressure campaign.

White House meeting with Ukrainian officials

Bolton participated in a White House meeting on July 10 with senior US and Ukrainian officials. In attendance were Sondland, Volker, Perry, Fiona Hill and Alexander Vindman from the National Security Council, and top Ukrainian officials Andrey Yermak and Oleksandr Danyluk.

During the meeting, Sondland told the Ukrainians they'd need to announce the investigations Trump wanted before Ukrainian President Volodymyr Zelensky would be invited to meet Trump in Washington, according to congressional testimony.

Some witnesses previously said Bolton abruptly ended the meeting after Sondland's comment, because he was improperly blending domestic politics with national security. Bolton's firsthand account of this meeting would be a crucial component of his potential testimony in the Senate.

Seeking distance from the 'drug deal'

Irritated after the meeting was derailed, Bolton vented privately to Hill, who was the National Security Council's top Russia expert at the time. Hill testified that Bolton instructed her to inform White House lawyers about the meeting, and to convey that Bolton was "not part of... whatever drug deal that (acting White House chief of staff Mick) Mulvaney and Sondland are cooking up."

This explosive testimony from Hill was a big part of her public appearance before the House Intelligence Committee last year. Bolton would likely be asked to explain his opposition to the "drug deal" and why he wanted to loop in the lawyers.

Hill also said Bolton privately expressed alarm about Trump's personal attorney Rudy Giuliani, who was also pushing Ukrainians to announce the investigations into Biden. If Bolton testifies, he would probably be asked to weigh in on Giuliani's role in the scheme, and it might not be pretty.

Bolton received updates from Volker

Last year, Volker testified that he told Bolton and other Trump administration officials "on various occasions" that he was "engaged in these conversations" with Ukrainian officials. Volker was at least partially referring to his contacts with Yermak about a public statement that Zelensky would make, about the investigations into Biden and election meddling in 2016.

It's not clear how much Bolton knew about these conversations, which Volker maintains were meant only to further US interests and not to assist Trump's reelection campaign. Volker and Sondland have maintained that they didn't realize that this public announcement could hurt Biden's 2020 presidential campaign -- Bolton's testimony could potentially undermine their denials.

Also, Hill testified that Bolton warned Volker "not to meet with Rudy Giuliani," though Volker apparently ignored this advice and met with Giuliani anyway in mid-July. Little is known about Bolton's conversations with Volker, and Bolton could have a chance to fill in some of the blanks.

Personal appeal to Trump on Ukraine aid

The New York Times reported that Bolton "made a personal appeal" to Trump on August 16 to lift the freeze on military aid for Ukraine. It's unclear who else witnessed this encounter and what was specifically stated. But it wasn't the last time that Bolton raised the issue with Trump.

Despite Bolton's pleas, Trump did not relent. The freeze remained in place until September 11.

Congressional Republicans have attacked the House impeachment inquiry for failing to uncover direct evidence of Trump's comments regarding Ukraine. ( Democrats say this is because Trump obstructed the inquiry.) Bolton's testimony about his August 16 conversation with Trump could reveal how Trump was explaining his decision to withhold the aid.

In-person meeting with Zelensky in Kiev

According to evidence uncovered during the House impeachment inquiry, some Ukrainian officials learned about the aid holdup as early as July. It is not known whether that information made its way up the chain to Zelensky, who later met with Bolton in Kiev on August 27.

The top US diplomat in Ukraine, Bill Taylor, accompanied Bolton during that meeting and later testified that the US security assistance was not discussed. The freeze wasn't widely known, and was not public, until it was revealed one day after the Kiev meeting in a Politico article

If Bolton testifies, he could confirm Taylor's testimony that he didn't talk about the aid holdup, which could help Trump's defense. Another US diplomat testified that Bolton and Zelensky discussed a planned meeting with Trump, which was later scrapped due to scheduling.

Chats with the US ambassador to Ukraine

After Bolton's meeting with Zelensky wrapped up, he privately met with Taylor and another US diplomat, David Holmes. Taylor said he expressed "my serious concern about the withholding of military assistance to Ukraine," and Bolton recommended that he send a first-person diplomatic cable to Secretary of State Mike Pompeo. Taylor followed through with the cable shortly after.

During this private meeting with US diplomats, Bolton expressed frustration about "Giuliani's influence with the President" and made it clear "there was nothing he could do about it," according to Holmes. This wasn't the first time Bolton was said to raise concerns about Giuliani.

Holmes also testified that Bolton said the freeze would only be lifted if Zelensky could "favorably impress" Trump at their planned meeting in Poland. It's not entirely clear what Bolton meant, and he could have just been talking about Zelensky's personality. But other officials said the aid would be lifted only after Zelensky convinced Trump he would announce the Biden probe.

Oval Office meeting with Trump on Ukraine

The situation escalated in late August, after the Politico article revealed the freeze on US aid. Bolton, Pompeo and Defense Secretary Mark Esper went directly to Trump in the Oval Office and pleaded with him to drop his hold on the US military aid, according to The New York Times

Their efforts fell flat, and Trump did not lift the hold at that time, despite the mounting pressure.

This Oval Office meeting was not mentioned during the impeachment inquiry in the House. If Bolton testifies, this could be the most important information he provides. (The White House might say these conversations are protected by executive privilege and can't be divulged.)

Unease with drastic moves to defend aid freeze

The New York Times also reported that Bolton opposed efforts by the White House budget office to devise a controversial legal justification for the unprecedented military freeze.

There is a law, the Impoundment Control Act , that requires presidents to spend money that is appropriated by Congress, which would include the nearly $400 million in military and security aid for Ukraine. According to The New York Times, White House lawyers believed Trump could ignore that law if he determined that releasing the money would undermine US diplomatic efforts.