Washington (CNN) Ukrainian President Volodymyr Zelensky, a political newcomer who took over in May, probably knew he would face all kinds of challenges during his first year in office.

A comedian-turned-politician who promised big changes , Zelensky would need to study the ins and outs of government. He would soon become commander-in-chief of a military still at war with Russia. And he'd need to start delivering on his campaign promise to root out corruption.

But as soon as Zelensky got to work on his reforms , he was pulled into an off-the-books push by President Donald Trump and his allies to start an investigation into Trump's political opponents.

For years, the US led the charge for Ukraine to clean up its own corruption. The former Soviet republic has struggled with elected officials who were more than happy to help friendly oligarchs and use their power to punish domestic political rivals.

But now, an American president was urging Zelensky to wade back into the muck of corruption.

"The idea that once you're in power, legal institutions are yours to use to remain in power is very familiar to Ukrainian politics," said Jordan Gans-Morse, a Northwestern University professor who was recently a Fulbright Scholar in Ukraine. "The only strange part is that the US president is playing a role that is typically played by a powerful person in Ukraine. That's the irony."

Rule of Law

Since the fall of the Iron Curtain, the US, European leaders and the International Monetary Fund have pressed Ukraine to make its judiciary truly independent as a prerequisite for much-needed aid. That effort ramped up considerably after the 2014 Ukrainian revolution cast out the country's Russian-backed government and brought in Western-minded leaders. Trump is now accused of withholding aid because Zelensky would not launch politically charged criminal investigations.

The George W. Bush administration touted $60 million in foreign aid to help Ukraine "establish the rule of law." The phrase became a common refrain from President Barack Obama throughout his second term. Secretary of State Mike Pompeo brought it up with his Ukrainian counterpart last year.

And so did former US Special Envoy for Ukraine Kurt Volker at a July 27 appearance in Kiev.

"President Zelenskyy has articulated a very clear commitment to reforms," Volker said. "There are so many areas to look at, but one of the most important is rule of law and judiciary."

But two days before those comments, Volker texted a top Zelensky adviser and dangled the possibility of a White House meeting in exchange for help investigating an unfounded conspiracy theory about the Democratic National Committee and Russian meddling in 2016.

"Heard from White House -- assuming President Z convinces trump he will investigate / 'get to the bottom of what happened' in 2016, we will nail down date for visit to Washington," Volker wrote.

Hours later, Trump turned the screws during a phone call where he asked Zelensky "to do us a favor" and investigate the DNC conspiracies, and "get to the bottom of" alleged corruption by former Vice President Joe Biden and his son. There is no evidence of wrongdoing by the Bidens. Trump's request came as Biden rose in the polls as a formidable 2020 challenger.

Pressure mounting

The pressure from the Trump administration had been mounting for months , even before Zelensky took office, CNN reported. A key player was Trump's personal attorney Rudy Giuliani, who raised the idea of investigating Biden in multiple television interviews and public tweets.

Zelensky and his team felt the heat in Kiev. According to texts from Ambassador Bill Taylor, the top US diplomat in Ukraine, Zelensky tried hard to navigate Trump's requests and wanted to avoid portraying his country as "an instrument in Washington domestic, reelection politics."

The US and Ukraine routinely cooperate on legal probes. To that end, Zelensky's advisers tried to go through proper legal channels. According to Taylor's testimony to Congress , Zelensky aide Andriy Yermak wanted the Justice Department to submit an official request for an investigation into Burisma, a Ukrainian energy company where Hunter Biden held a board seat since 2014.The US and Ukraine routinely cooperate on legal probes.

There is no indication that the Justice Department ever received or granted that request.

"If there was a legitimate law enforcement matter to pursue, there is actually a legal attaché at our embassy whose whole job for being posted overseas is to pursue an investigation that the United States would like to see the Ukrainians take up," said Brett Bruen, a 12-year foreign service officer who worked in the Obama administration. "That person was not part of this process."

'About-face of US policy'

As Zelensky dragged his feet on giving into Trump's demands, the entire episode burst into public view in September, after a whistleblower complaint about Trump's actions was released to the public. The White House also released its transcript of Trump's phone call with Zelensky.

The uproar triggered a wave of support among House Democrats for an impeachment inquiry, which was promptly launched by House Speaker Nancy Pelosi. The inquiry has heated up in recent weeks, with subpoenas flying out and witnesses coming to Capitol Hill to tell their story.

And Pelosi has ratcheted things up with Trump directly, telling him that "all roads seem to lead to (Russian President Vladimir) Putin" during an intense White House meeting last week.

"Not only is this an about-face of US policy, but it also plays directly into Russia's hand," Bruen said. "Russia uses this corruption to manipulate events in Ukraine. Their argument all along was that the US is no better than they are. And President Trump has essentially proven them right."