WASHINGTON — Military aid promised by the U.S. to Ukraine — and the strange circumstances under which it was held up and eventually released — is at the heart of the impeachment inquiry into President Donald Trump.

For Republicans, the key fact is that Ukraine received the money, regardless of any request from Trump for an investigation of Joe Biden or the 2016 U.S. elections. For Democrats, withholding the aid for investigations is an abuse of power, regardless of what happened in the end.

A look at key dates involving the nearly $400 million in military assistance that had been approved for release in the early months of 2019:

JULY 3: The hold

Lt. Col. Alexander Vindman, a national security official working at the White House, becomes aware that the military aid has been held up. He testified that he received a notice from the State Department. “That’s when I was concretely made aware of the fact there was a hold placed,” he said in testimony to lawmakers.

JULY 10: The meeting

A meeting at the White House with Ukrainian officials is cut short when Gordon Sondland, the U.S. ambassador to the European Union, says he has an agreement with the acting White House chief of staff that Ukraine’s president would get a meeting with Trump if Ukraine agreed to launch investigations.

Then-national security adviser John Bolton “stiffened” and ended the meeting, later telling colleague Fiona Hill to report it to the National Security Council’s lawyer, she testified.

“I am not part of whatever drug deal Sondland and (acting White House chief of staff Mick) Mulvaney are cooking up on this,” Hill said Bolton told her.

JULY 18: The hold-up announcement

In a secure call with national security officials, a staff member of the White House Office of Management and Budget announces there’s a freeze on Ukraine aid until further notice, based on a presidential order to the budget office.

JULY 25: The phone call

Trump speaks with Ukrainian President Volodymyr Zelenskiy, asking him for favors that include an inquiry into Joe Biden’s son Hunter’s dealings with Burisma, a Ukrainian gas company, and to investigate whether Ukraine interfered in the 2016 U.S. presidential elections. He later calls it a “perfect” call.

AUGUST: The questions

Catherine Croft, the special adviser for Ukraine at the State Department, says two Ukrainians reach out to her to ask about the status of the military assistance. She told lawmakers she couldn’t recall the exact dates, but believes the outreach took place before the Aug. 28 publication of a Politico article detailing the hold.

AUG. 12: The complaint

A whistleblower files a formal complaint addressed to Congress that details concerns over the July 25 phone call and the hold placed on the military aid. The complaint is withheld from Congress until Sept. 25.

AUG. 28: The article

Politico publishes details that the military aid to Ukraine is on hold, setting off a scramble among diplomats in Ukraine and the United States.

AUG. 29 AND AFTER: Ukraine’s desperation

William Taylor, the acting U.S. ambassador to Ukraine, testified that he did not know the aid had been withheld until after the Politico article appeared, when he started receiving “desperate” calls from Ukrainian officials.

“The minister of defense came to me,” he said. “I would use the word ‘desperate,’ to try to figure out why the assistance was held.”

Taylor said the minister thought if he spoke to Congress, or the White House, he could find out the reason and reassure them of whatever was necessary to get the aid. If the money wasn’t provided by Sept. 30, it would be lost.

SEPT. 9: The investigations begin

Three House committees launch a wide-ranging investigation into the allegations that Trump, his personal lawyer Rudy Giuliani, and possibly others, tried to pressure the Ukrainian government to help the president’s reelection campaign by digging up dirt on a political rival.

SEPT. 11: The aid is released

The funds are suddenly released. Senate Republicans said that happened in part because Sen. Dick Durbin, an Illinois Democrat, threatened to block $5 billion in Pentagon spending for 2020 if the aid wasn’t given to Ukraine. They said the aid was held up while Trump looked into whether Zelenskiy was serious about fighting corruption. Taylor and other diplomats involved in Ukraine were not given a reason for the aid being released.

IN THE AFTERMATH: The canceled CNN interview

Taylor said Ukraine’s president was planning to do an interview with CNN in which he would make a public statement on the investigations that Trump had pushed for.

Taylor was concerned about the interview and its potential to play into “domestic U.S. politics,” and on Sept. 13 asked Ukrainian officials about it. The interview never happens.