WASHINGTON – A string of U.S. diplomats and national-security officials mapped out during the last two weeks for the House impeachment inquiry how President Donald Trump's pressure on Ukraine to investigate his political rival began months earlier than a key July 25 call and raised alarms across the government.

The U.S. ambassador to Europe voiced disappointment after learning May 23 that the president delegated Ukraine policy to his personal lawyer, Rudy Giuliani, rather than rely on professionals at the State Department.

Trump’s top diplomat in Ukraine, who caught wind of Giuliani’s back-channel effort, said that trading military aid to Ukraine for an investigation of corruption would be “crazy.”

And John Bolton, the president’s former national security adviser, called Giuliani a “hand grenade” that would blow up routine diplomatic efforts, according to a former National Security Council official.

The efforts culminated in the call July 25 between Trump and Ukraine President Volodymyr Zelensky. Trump acknowledged urging an investigation of former Vice President Joe Biden and his son Hunter by releasing a summary of the call. “It sounds horrible to me,” Trump said to Zelensky about the Bidens.

Mick Mulvaney:Mick Mulvaney acknowledges Trump held up aid to pressure Ukraine, then rows back

Biden, a leading Democratic presidential contender in 2020, has denied wrongdoing. Ukrainian officials have said there was no basis for allegations against the Bidens.

Witnesses before the three House panels – Foreign Affairs, Intelligence, and Oversight and Reform – focused the impeachment spotlight on a central question: Was Trump justified in withholding nearly $400 million in military aid from Ukraine in exchange for an investigation of corruption, or did that demand qualify for removing the president from office? The answer among lawmakers broke down sharply along partisan lines.

Trump insisted he was absolutely justified in fighting corruption in Ukraine. Republican lawmakers defended Trump for fighting corruption while not explicitly demanding an investigation of the Bidens in exchange for military aid during the call.

“I am personally of the opinion that our country should absolutely be working together to go after corruption, and the Biden family scheme in Ukraine is one that my constituents have an issue with, the president of the United States has an issue with, we have an issue with,” Rep. Lee Zeldin, R-N.Y., said Tuesday.

Acting White House chief of staff Mick Mulvaney said Thursday the military aid was withheld in exchange for Ukraine investigating the hacking of a Democratic National Committee computer server in 2016, and to assist a Justice Department review of that election interference, rather than targeting the Bidens. Mulvaney later issued a statement denying any quid pro quo exchanging military aid for an investigation. But in front of reporters, Mulvaney said trade-offs are common in diplomacy.

“Get over it,” Mulvaney told reporters, in a message the Trump campaign began selling Friday on T-shirts.

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But House Democrats said Trump’s demand for an investigation – while naming the Bidens as targets – at the same time he withheld military aid could be grounds for impeachment. The administration’s defiance of congressional subpoenas for documents and testimony could also be grounds for impeachment as examples of obstruction of justice.

"The reality is, as much as there's a lot of focus on a quid pro quo, I just want to remind everyone, you do not need a quid pro quo," Rep. David Cicilline said. "The United States asking a foreign leader to interfere in an American presidential election is illegal, un-American, unpatriotic and in it of itself constitutes grounds for impeachment."

House Intelligence Committee Chairman Adam Schiff, D-Calif., called Mulvaney's admission "a phenomenal breach of the president's duty to defend our national security."

Following is a summary of highlights from the past three weeks of impeachment events:

Texts outline diplomatic concerns

Kurt Volker, the former U.S. special envoy for Ukraine, told the House inquiry Oct. 3 that Trump was skeptical in a May 23 briefing that Ukraine was serious about fighting corruption and deserved robust U.S. support.

"He said that Ukraine was a corrupt country, full of 'terrible people,'" Volker said. "He said they 'tried to take me down'.”

Texts between U.S. diplomats questioning Trump’s Ukraine policy were released as part of Volker’s testimony.

"Are we now saying that security assistance and WH meeting are conditioned on investigations?" William Taylor, the top U.S. diplomat in Ukraine, asked in a text message Sept. 1 to Volker and Gordon Sondland, the U.S. ambassador to the European Union.

"Call me," Sondland texted back.

Taylor texted again Sept. 9: "As I said on the phone, I think it's crazy to withhold security assistance for help with a political campaign."

Sondland later said he called Trump before responding that the assertion was “incorrect.”

“The President has been crystal clear no quid pro quo’s of any kind,” Sondland said in a text message.

Ambassador opposed Giuliani's role

Sondland testified Thursday that Trump seemed uninterested in Ukraine policy and told the ambassador to consult with Giuliani instead.

"It was apparent to all of us that the key to changing the president's mind on Ukraine was Mr. Giuliani," Sondland said. "Our view was that the men and women of the State Department, not the president's personal lawyer, should take responsibility for all aspects of U.S. foreign policy toward Ukraine."

Sondland said he wasn’t aware that corruption investigations were targeting the Bidens or that military aid was withheld, but he said both steps would have been wrong. Sondland said he didn’t participate in the July 25 call and only learned the details when the summary was released Sept. 25.

"Let me state clearly: Inviting a foreign government to undertake investigations for the purpose of influencing an upcoming U.S. election would be wrong," Sondland said. "Withholding foreign aid in order to pressure a foreign government to take such steps would be wrong. I did not and would not ever participate in such undertakings."

Giuliani called a 'hand grenade'

Fiona Hill, the National Security Council's former senior director for Europe and Russia, told the House inquiry Oct. 14 that John Bolton, the former national security adviser, said he wasn’t part of whatever deal Sondland and Mulvaney were “cooking up.”

Bolton also referred to Giuliani as "a hand grenade who’s going to blow everybody up,” Hill said.

Giuliani told NBC News that he had always liked and respected Bolton, but that he was “very disappointed that his bitterness drives him to attack a friend falsely and in a very personal way."

State Department morale

Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo, told the House panel Wednesday that he quit in late September in solidarity with career diplomats frustrated with Giuliani’s back-channel diplomacy.

Marie Yovanovitch, the former U.S. ambassador to Ukraine, told the House panel Oct. 11 that the State Department removed her from the post in May under pressure from Trump.

A high-ranking official told her "that the president had lost confidence in me and no longer wished me to serve as his ambassador," she said. "He also said that I had done nothing wrong and that this was not like other situations where he had recalled ambassadors for cause.”

Impeachment probe:Ex-Pompeo aide testifies about Ukraine as spotlight shines on Trump's foreign policy

Trump called her “bad news” during the call with Zelensky. Republican lawmakers said Trump has the authority to name or remove any ambassadors representing the country.

Pompeo disputed allegations that he failed to defend career diplomats such as Yovanovitch."I protect every single State Department employee," Pompeo told The Tennessean. "It’s one of the reasons that we asked the House of Representatives to stop their abusive prosecutions where they won’t let State Department lawyers sit with our employees. That’s not fair."

Giuliani associates arrested

The arrest Oct. 9 of two Giuliani associates, who introduced him to a key Ukrainian prosecutor in January, revealed how they had cultivated Republican contacts more than a year before Trump’s call July 25.

The pair of business partners – Ukrainian-born Lev Parnas and Igor Fruman from Belarus – contributed generously to Republican political groups and dined with Trump at the White House in May 2018. But they were arrested at Dulles International Airport clutching one-way tickets to Germany and were charged with campaign-finance violations. They pleaded not guilty on Thursday.

U.S. counterintelligence agents have been examining Giuliani’s business dealings with Parnas and Fruman since at least early 2019, according to a Manhattan lawyer, Kenneth McCallion, who has represented several Ukrainian clients.

Giuliani:Federal investigators have been looking into Giuliani's dealings in Ukraine since early 2019

Trump denied knowing the two men, despite hosting them at the White House. Giuliani declined comment on their arrest.

In a related move, House investigators set a Friday deadline for documents from Energy Secretary Rick Perry because he urged Trump to call Zelensky. The House panels also cited reports that Perry suggested Ukraine to remove board members from a state-owned energy company and replace them with Parnas and Fruman.

But the White House notified lawmakers Oct. 8 that it wouldn’t cooperate with subpoenas because the House hasn’t authorized the impeachment inquiry. The Energy Department notifies the House inquiry Friday it wouldn’t comply with the subpoena “at this time.”

Perry announced his resignation Thursday, saying he would leave office later this year.