The larger war on Ukraine has underscored the importance of U.S. military aid to help blunt Russia's invasion and prevent the conflict from spilling into NATO countries. One of the two articles of impeachment against Trump accused him of ordering nine senior aides to refuse to testify, as well as the Trump administration's refusal to cooperate with the Democrat-led impeachment investigation.

Key accounts at the heart of the controversy have never been revealed by those aides. Those who were involved in holding back the aid say there is no need for further investigation.

Mark Paoletta, who was general counsel at the Office of Management and Budget at the time, told POLITICO that the hold did not cause any lapsing of funds.

The Obama administration refused to give lethal aid to Ukraine because they didn't want to offend Putin. The Trump administration began to supply the missiles.

Among the nine aides who put up stiff resistance to the impeachment inquiry were Russell Vought, who served as Trump's budget director, and Michael Duffey, who was his deputy. The Ukraine aid was put on hold and investigators ended up with a lot of unanswered questions. The withholding of aid was found to be illegal by a federal watchdog.

The Senate, mostly along party lines, acquitted Trump of the House charges that he sought to extort a foreign leader for political gain and then obstruction of Congress. Less than a week after the Senate acquitted Trump on both counts, he testified before the House Budget Committee. Democrats never asked him about the initial decision to stop military assistance.

Vought was willing to testify before the impeachment investigators if an administration lawyer was present to protect claims of executive privilege, but Democrats rejected the offer.

Aid from the U.S. and Western nations has provided a lifeline to Zelenskyy's government, which is one of the reasons for the current war.

One person involved in the first impeachment probe, who spoke candidly on condition of anonymity, pointed to a question left by the refusal of some of Trump's budget advisers to testify or share documents: the contingency plan drawn up by defense officials to deliver Ukraine's aid. Even though planning documents may have been drafted, the backup plan was never set in motion because the aid was delivered.

The impeachment inquiry showed that Trump tried to get diplomats and advisers to help him in his campaign to get Zelenskyy to investigate the Biden family. Rudy Giuliani, the former New York City mayor and Trump's personal attorney, routed most of the bid.

Zelenskyy tried to get a White House visit, but was repeatedly turned down by Trump, who discouraged Mike Pence from attending the Ukrainian leader's inauguration as a sign of U.S. support. There is a mystery surrounding a Sept. 18, 2019, phone call that he, a key Trump envoy to Ukraine, had with Zelenskyy.

The call came days after the aid to Ukraine was released and as impeachment fervor began to take root among House Democrats. Democrats protested that the decision to shield the information was unwarranted.

The House Intelligence Committee was told about the Pence-Zelenskyy call by a national security aide to the vice president. After her public testimony, Williams recalled the phone call. The nature or substance of his conversation with Zelenskyy was not answered by the aides.

Democrats insist that Trump bears some of the responsibility for the current crisis in Ukraine, despite holes in the public's understanding of the Trump-Ukraine saga. Democrats say that the former president's willingness to condition support on political investigations signaled to Putin that the west wouldn't be united behind him.

Warner said that Trump's decision to deny aid made the Ukrainians less prepared to fight the Russians.

Warner said that the guy tried to extort political favors from Zelenskyy for his own personal political gain.

The figures involved in the affair still have questions about what happened. Marie Yovanovitch, who was ousted by Trump just before Zelenskyy's inauguration, told POLITICO in a recent interview that she believed Trump's treatment of Ukraine. She said that a string of tell-all books have shed new light on the events that led to her removal.

Yovanovitch said that there will be more details forthcoming.

This report was contributed to by two people.