KYIV — As soon as he got the invitation from Rudy Giuliani, President Trump’s personal lawyer, it was abundantly clear to him what Trump’s allies were after.

“I understood very well what would interest them,’’ Yuriy Lutsenko, Ukraine’s recently fired prosecutor general, said in an extensive interview in London. “I have 23 years in politics. I knew.’’

“I’m a political animal,’’ he added.

When Lutsenko sat down with Giuliani in New York in January, he recalled, his expectations were confirmed: The president’s lawyer wanted him to investigate former vice president Joe Biden and his son Hunter.

It was the start of what both sides hoped would be a mutually beneficial relationship — but one that is now central to the impeachment inquiry into Trump.

Trump and his allies have been fixated on Ukraine since the 2016 US election, convinced that the country holds the key to unlock what they view as a conspiracy to undermine Trump. Giuliani in particular has viewed Ukraine as a potentially rich source of information beneficial to Trump and harmful to his opponents, including Biden.

There is no evidence of wrongdoing by Biden or his son in Ukraine. On Saturday, Secretary of State Mike Pompeo defended the administration’s actions, saying it “was incredibly focused on making sure that we worked with Ukraine in a way that was appropriate.’’

But a detailed look at Lutsenko’s record shows how Trump and his allies embraced and relied on a Ukrainian prosecutor with no formal legal training and a long history of wielding the law as a weapon in his personal political battles, disregarding the concerns of senior diplomats who said he wasn’t credible.

Trump praised him in a phone call with Ukraine’s president. Giuliani aggressively promoted the news that Lutsenko’s office had revived an investigation into the owner of a Ukrainian energy company that had hired Biden’s son. And in an interview with Fox News in April, Trump described Lutsenko’s claims as “big’’ and “incredible,’’ worthy of attention from the US attorney general.

Trump’s allies even seemed to favor Lutsenko over the US ambassador in Ukraine, who was recalled as the president’s supporters stepped up pressure on the country to investigate the Bidens.

In the impeachment debate, Ukraine has often seemed an innocent bystander, a poor and deeply troubled country on Europe’s eastern fringe sideswiped by the raucous political battles of the world’s most powerful nation.

But the scandal now roiling Washington underscores how Ukraine’s own domestic struggles, feuds, and dysfunctions have shaped the controversy — and how the pursuit of political advantage by actors in each country fed the other in ways that neither side foresaw.

Lutsenko’s path to Giuliani began in this political morass, with a meeting so combative that it helped ignite the scandal in the first place.

Shortly after taking up her post in 2016, the US ambassador to Ukraine, Marie L. Yovanovitch, went to meet the new prosecutor general, Lutsenko, in his office — and complained that his deputies were stained by corruption, according to two Ukrainian officials familiar with the encounter.

The ambassador then pressed Lutsenko further, the officials said, asking him to stop investigating anti-corruption activists who were supported by the US Embassy and had criticized his work.

Lutsenko said he snapped at Yovanovitch that “no one is going to dictate to me’’ who should be investigated, prompting the ambassador to storm out of the meeting.

In the months to come — as the ambassador stepped up her criticism of Ukraine’s faltering efforts to root out corruption — Lutsenko’s personal animus toward Yovanovitch grew. He concluded, he and his former colleagues say, that he needed to go around her and find a direct path to a more receptive audience: Trump’s inner circle.

When Giuliani learned that Lutsenko and other disgruntled Ukrainian officials were trying to reach out to the Americans, he welcomed the opportunity.

“Yeah, I probably called, I’m sure I called — Lutsenko didn’t have my number,’’ Giuliani said in an interview.

According to notes of their January meetings given to members of Congress last week, Lutsenko told Giuliani about what he called payments to Hunter Biden, who sat on the board of the Ukrainian energy company Burisma.

The two also discussed the theory that Paul Manafort — Trump’s former campaign manager, who had been convicted in the United States of fraud for his work as a consultant in Ukraine — had been set up by supporters of Hillary Clinton. Ukrainian officials deny such claims, and no evidence supports this idea.

Lutsenko said he met Giuliani to seek help recovering billions of dollars he said was stolen from Ukraine under a previous government, a matter unrelated to the US election.

But veterans of Ukraine’s cutthroat politics say Lutsenko’s outreach to Trump’s inner circle was a clear attempt to win favor with a powerful ally at a time his own political future looked uncertain.

“Lutsenko was trying to save his political skin by pretending to be Trumpist at the end of his career,’’ said David Sakvarelidze, a former deputy prosecutor general.

Instead of finding salvation, Lutsenko was fired in late August by Ukraine’s new president, Volodymyr Zelensky.

Lutsenko left Ukraine for Britain last Sunday, saying he wanted to improve his English. On Tuesday, Ukrainian authorities announced that they had opened a criminal case against him over accusations that he had abused his power in dealings with politicians and others involved in illegal gambling.

Lutsenko dismissed the latest case as “a big fantasy.’’ But to many in Ukraine, it is a fitting coda to the career of an ambitious politician turned prosecutor who used his position to wage political battles.

Even his initial appointment caused controversy: He became prosecutor general in 2016 only after Ukraine’s president at the time, Petro O. Poroshenko, got Parliament to remove a requirement that the prosecutor be educated in the law.

A survivor in Ukraine’s often treacherous politics, Lutsenko had spent time in jail as a political prisoner, won a seat in Ukraine’s Parliament and served as interior minister, holding senior positions under three presidents.

He also showed himself an adept operator in the United States.

After his meetings with Giuliani, Lutsenko provided grist for a series of articles in The Hill, a Washington news portal. His remarks were pitch perfect in their appeal to Trump and his supporters.

In one article, Lutsenko aired his feud with Yovanovitch, the US ambassador, asserting that she had given him a list of untouchables not to prosecute. The claim set off a storm of accusations that the ambassador belonged to a cabal working to hurt Trump and protect the Bidens.

The State Department dismissed Lutsenko’s claim as “an outright fabrication,’’ and he later acknowledged that the “don’t prosecute list’’ never existed.

In the interview, he blamed the misstep on a bad translation and insisted that Yovanovitch had, in fact, pressed him not to prosecute anti-corruption activists.

But the damage was done. Already under fire from some Republicans, who said she had disparaged Trump in private meetings, Yovanovitch was ordered in May to leave her post in Kyiv and return to Washington.