WASHINGTON – From the details of the “perfect” call to allegations of a shadow foreign policy, the impeachment proceedings provided an unprecedented view of the inner workings of President Donald Trump's White House.

The inquiry unearthed a trove of text messages, testimony and documents from players in the Trump administration's campaign to pressure Ukraine to investigate former Vice President Joe Biden and his son Hunter, who worked for a Ukrainian energy company.

But the House investigation and the Senate trial left many questions unanswered.

Here are five things we learned, and didn't learn, since a whistleblower complaint triggered a four-month political drama that engulfed Washington:

What we learned: ‘No guardrails’ on Trump’s foreign policy

The impeachment proceedings pulled back the curtain on Trump’s approach to foreign policy – revealing a chaotic tangle of competing agendas, questionable motives and confusion about who called the shots.

"What we learned was how the sausage is made in the Trump foreign policy shop," said Brett Bruen, a foreign service officer who served as the global engagement director in the Obama administration. Where President Barack Obama's foreign policy was driven by wide-ranging, wonky debates among experts, Trump's seems to be driven by "unconventional actors" and an impulsive president, he said.

A fresh example: On an audiotape released in late January, Trump is heard ordering the removal of U.S. Ambassador to Ukraine Marie Yovanovitch. The president was not talking to his Secretary of State Mike Pompeo but to Lev Parnas, an associate of his personal lawyer Rudy Giuliani, who blasted Yovanovitch as disloyal.

Parnas, a Trump supporter with no diplomatic experience, was "essentially providing the president this unsubstantiated, unfiltered view of what’s going on in Kiev, and he’s buying it," Bruen said. Parnas and his associate Igor Fruman face campaign finance charges in New York.

Fiona Hill, Trump’s former national security adviser for Europe and Russia, painted a vivid picture of White House disarray. Last fall, Hill testified that she was shocked to learn that Gordon Sondland, Trump’s ambassador to the European Union, was put in charge of Ukraine policy, an unusual portfolio for him since Ukraine is not part of the EU. She said he didn't appear to be receiving the required briefings for meetings he was holding with foreign leaders.

“It's like basically driving along with no guardrails and no GPS on an unfamiliar territory,” she said. Sondland gave out his cellphone number – and hers – to foreign officials whose communications were vulnerable to Russian espionage, she said.

“We had all kinds of officials from (foreign governments) … literally appearing at the gates of the White House, calling on our personal phones,” she recounted. “I'd find endless messages from irate officials who'd been told that they were supposed to meet with me by Ambassador Sondland."

What we didn't: Do Trump's allies still seek help from Ukraine?

Despite the spotlight on Ukraine, the Trump administration's policy toward Kiev remains pocked with uncertainty.

Ukraine is at war with Russia, a U.S. adversary, and needs American support to ward off Moscow's attacks on its sovereignty. Friday, Secretary of State Mike Pompeo landed in Kiev and promised unwavering U.S. support for Ukraine.

Pompeo did not invite Ukrainian President Volodymyr Zelensky to the White House – a visit the Ukrainian leader sought to bolster his global standing as he prepared to confront Russian President Vladimir Putin.

"We’ll find the right time," Pompeo said, adding that Zelensky would be welcome "when we have an opportunity to do good things for both the Ukrainian people and the American people."

Even as Pompeo said there are no conditions for a meeting, Giuliani has continued to pursue his investigation of Joe and Hunter Biden, who was on the board of Ukrainian gas company Burisma when his father was vice president. Giuliani traveled to Kiev in December, talking to unidentified officials about the Bidens.

Trump and Giuliani promoted allegations that Biden, while vice president, sought to have a Ukrainian prosecutor fired so he would not investigate Hunter and Burisma. Biden said he helped oust the prosecutor because he was widely viewed as corrupt, and no evidence of wrongdoing by Hunter Biden has been uncovered.

A slew of Ukraine experts have left the Trump administration since the allegations against Trump emerged, creating a diplomatic vacuum.

What we learned: Giuliani operated as Trump’s shadow secretary of state

Giuliani may not have an official diplomatic title in Trump’s State Department, but the impeachment proceedings revealed his outsized influence on Ukraine matters.

He reached out directly to Zelensky and dispatched associates to carry out Trump’s Ukraine pressure campaign.

Sondland, Trump’s ambassador to the EU, said he worked with Giuliani at "the express direction of the president" to pressure Ukrainian officials for political favors.

The evidence showed that Giuliani used official State Department channels – including Trump’s former special envoy to Ukraine Kurt Volker. He also relied on two nondiplomats, Fruman and Parnas, to carry out elements of the campaign.

What we didn’t learn: Did Pompeo try to stop Giuliani – or did he enable him?

Pompeo has said little about how much he knew of the Ukraine pressure campaign and what role he played, if any, in executing it, though other officials contend he was fully aware and may have helped facilitate some of the efforts by Giuliani and others.

Sondland said he kept Pompeo and other State Department officials “in the loop” about Ukraine.

The diplomat produced emails to Pompeo and other top Trump administration officials, showing he communicated with them regularly about his efforts.

The State Department released records showing Pompeo had two phone calls with Giuliani in late March as Giuliani ramped up pressure on Ukrainian officials to open the Biden investigations.

"I don't have much to say with respect to the Ukraine investigation," Pompeo said in November when asked about his conversations with Giuliani.

Aaron David Miller, a senior fellow at the Carnegie Endowment for International Peace, said the level of Pompeo's involvement remains a mystery.

“But the more we learn – drip, drip, drip – the more it appears that Pompeo played a bigger role (rather) than a smaller one," said Miller, who has advised Republican and Democratic presidents on U.S. foreign policy.

What we learned: GAO says Trump violated law in withholding Ukraine aid

In January, the Government Accountability Office, a nonpartisan government watchdog, concluded the White House broke the law when it withheld nearly $400 million in military aid to Ukraine, a finding the White House rejected.

Democratic lawmakers cited the report in arguing that Trump abused his power in his dealings with Ukraine. The Impoundment Act governs Congress’ constitutional power of the purse, but violations do not carry criminal penalties.

What we didn’t learn: Did Trump commit a crime?

Trump’s legal team called the impeachment case "structurally deficient," saying neither of the two articles of impeachment – abuse of power and obstruction of Congress – are violations of federal criminal statutes. Democrats said the language in the Constitution suggests that abuse of power was at the heart of why the founders included an impeachment clause in the document.

Randall Eliason, who teaches white-collar criminal law at George Washington Law School, argued that the articles against Trump include the federal language of bribery even without defining it as such.

"By making the allegations under the abuse of power they avoided getting bogged down in the weeds of the precise requirements of any particular federal statute," Eliason said. "But the flip side of that is this Republican argument that no president has ever been impeached for something that isn't even a crime."

What we learned: The whistleblower’s complaint has been corroborated by testimony

The president’s impeachment was set off by an anonymous whistleblower complaint lodged Aug. 12 to the intelligence community's inspector general, accusing Trump of abusing his power to pursue investigations that politically benefited him.

The complaint centered on Trump’s call with Zelensky on July 25, when the U.S. president urged his counterpart to announce the Biden investigations. The nine-page complaint alleged the Trump administration carried out a months-long pressure campaign in Kiev that subverted regular U.S. foreign diplomatic channels.

The whistleblower, identified only as an intelligence community official, is protected from retaliation by a federal statute shielding employees or contractors who want to report wrongdoing by the government.

The president and his defenders argued the account relied on secondhand information, but the complaint was largely corroborated through testimony from 18 witnesses, including several foreign service officials and career diplomats, during the House impeachment inquiry.

How we got here: A visual timeline of the events that led up to Trump's fateful phone call

Acting Ukraine Ambassador William Taylor told lawmakers he “became increasingly concerned that our relationship with Ukraine was being fundamentally undermined,” and Alexander Vindman, the top Ukraine expert on the National Security Council, testified he sounded the alarm about the pressure campaign to the agency’s lead counsel.

Hill, the former NSC senior director for Europe and Russia, told Congress that national security adviser John Bolton was furious about the push to solicit political investigations from Ukrainian officials. According to New York Times reports about Bolton's forthcoming book, the president directly told his national security director that he was withholding aid in exchange for the investigations.

After the complaint’s release, the White House released a rough summary of the call in which Trump urged Zelensky to open inquiries into the Bidens. Trump described the call as "perfect."

Republicans demanded the whistleblower testify in the impeachment process, but Democrats argued that wasn't needed because the account was corroborated by other witnesses.

What we didn't learn: The whistleblower’s identity

Republicans, including the president, made several attempts to out the individual, accusing House Intelligence Chairman Adam Schiff, D-Calif., of coordinating with the whistleblower to hatch the impeachment drive.

Schiff denied knowing who the person is and rejected allegations that his committee helped write the complaint or coached the whistleblower.

Donald Trump Jr., the president's son, tweeted the name of a person conservative websites identified as the whistleblower, but USA TODAY has not independently verified those allegations.

In late December, the president, who attacked the whistleblower on Twitter and at campaign rallies, came under fire for retweeting a post naming the individual and suggesting the person committed perjury.

Sen. Rand Paul, R-Ky., is among the most vocal critics of the whistleblower and demanded that the individual’s name be publicly released. He’s mentioned the alleged whistleblower’s name in media interviews over the past few months.

The Kentucky senator again attempted to reveal the alleged whistleblower’s identity Jan. 30 when he submitted a question naming the individual to Supreme Court Chief Justice John Roberts, who presided over the Senate impeachment trial.

Roberts said he would not read the senator’s question explicitly naming the alleged whistleblower. Paul expressed frustration in a news conference after arguing whistleblower anonymity protections did not apply to Trump’s case.

What we learned: Trump froze military aid as he and Giuliani pressured Ukraine

The impeachment evidence shows that on July 25 – the same day as Trump’s call with Zelensky – the White House froze American security assistance to that country.

Democrats said Trump used the prospect of a White House visit for Zelensky and the U.S. assistance, which Ukraine needed to fight Russian attacks on its sovereignty, as leverage to force Zelensky to open the investigations.

Before the Trump-Zelensky call, Trump's special envoy for Ukraine, Kurt Volker, texted an aide to Zelensky: "Assuming President Z (Zelensky) convinces trump he will investigate/'get to the bottom of what happened' in 2016, we will nail down a date for visit to Washington. Good luck!"

"Those words couldn’t be much clearer," Schiff said during the Senate trial. He noted that during the call, Trump asked Zelensky to investigate the Bidens and an allegation that Ukraine interfered in the 2016 election.

Trump and his GOP allies said the president halted the aid temporarily over concerns about corruption in Ukraine. Trump's lawyers argued that during the call, Trump did not make a direct link between the U.S. aid and investigations.

Trump's team argued that the president released the U.S. aid even though Ukraine did not announce the investigations.

"An explicit quid pro quo for alleged improper campaign interference would've had President Trump saying to his counterpart in Ukraine, 'Here's the deal,' and followed up by explicitly linking a demand for an investigation of the Bidens to the provision or release of foreign aid," attorney Robert Ray said.

What we didn’t learn: Was that a quid pro quo?

As the impeachment proceedings unfolded, two Trump officials said there was a direct link between the aid Ukraine needed and the investigations Trump wanted. Sondland, Trump’s ambassador to the European Union, said he came to the conclusion that Trump withheld the aid – as well as a White House visit for Zelensky – as part of a “quid pro quo.”

Sondland said he did not have firsthand knowledge that Trump made that link directly.

Mick Mulvaney, Trump's acting chief of staff, acknowledged the aid was withheld in part because the president wanted Ukraine to probe alleged meddling in the 2016 presidential election, specifically the assertion that a hacked Democratic National Committee server was in Ukraine.

"Let me be clear, there was absolutely no quid pro quo between Ukrainian military aid and any investigation into the 2016 election," Mulvaney said in a later statement. "The president never told me to withhold any money until the Ukrainians did anything related to the server."

Trump's attorneys said the Democrats relied on assumptions made by Sondland and others about the president's intent. Democrats failed to secure testimony on the pivotal quid pro quo question from the most tantalizing witness: Bolton, Trump’s former national security adviser, who hinted that he could offer a firsthand account of the president’s demands on Ukraine.

Bolton remained a wild card throughout the impeachment proceedings – a star player, if a (mostly) silent one.

The New York Times reported that Bolton, in the draft of a memoir, said Trump wanted to maintain the Ukraine aid freeze until Zelensky agreed to investigations of the Bidens. The Times reported that Bolton’s book includes an account of Trump directly asking Bolton to pressure Ukraine for the Biden investigations.

Last week, the Senate voted against calling witnesses, including Bolton, to testify in the trial – leaving open the question of what he would have told lawmakers. His book, “The Room Where It Happened,” is scheduled for release in March.

Democrats warned that the absence of witnesses was a mistake that could haunt Republicans.