Washington (CNN) For Rudy Giuliani , the final week of September could very well go down as the most consequential stretch of days in his long public career since he led New York City through the tragedy of 9/11.

Things started benignly enough. On Friday night, September 20, the President's personal attorney attended a black-tie state dinner at the White House , smiling for cameras alongside his girlfriend Maria Ryan. Yet a week later, Giuliani was under siege, and at the heart of events that have led to the first formal impeachment inquiry into a president in 20 years.

In between, Giuliani conducted a one-man messaging war, going on Fox News at least four times, talking to numerous reporters, and sending more than 50 tweets, many of which were about Joe Biden, his son Hunter, and their alleged dealings in Ukraine.

It was a surreal climax for Giuliani's 10-month, quixotic mission aimed at proving his theory that the origins of the investigation into Russian election interference can be traced back to Democrats' dealings in Ukraine. Along the way he went to Warsaw, Madrid, and very nearly Ukraine. He met with foreign nationals, coordinated with US State Department officials, dropped vague, confusing accusations in cable-TV appearances, and ceaselessly pitched US journalists on what he claims is one of the biggest scandals in modern American history— all while whispering in Donald Trump's ear about how Ukraine contained the keys to revealing truths about the President's political rivals.

The loose, slipshod strategy was designed to push a counternarrative about the true origins of the Russian investigation. ( Stirring up trouble for Biden was an added benefit.) Instead, in a dizzying reversal, Giuliani's actions boomeranged back on the President this week with stunning speed. By Friday, not only were Trump and Giuliani's Ukraine dealings the focus of the impeachment inquiry , but Secretary of State Mike Pompeo had been subpoenaed and the top US diplomat to Ukraine had resigned

Giuliani himself now faces the prospect of being in the crosshairs of the House's impeachment investigation.

Speaking Friday with CNN's Jim Sciutto, Rep. Mike Quigley, an Illinois Democrat on the House Intelligence Committee, did not rule out calling on Giuliani to testify or even subpoenaing him. "I obviously have several questions for him," Quigley said.

Giuliani said he would not testify without consulting his client, Trump, and that testimony about his work for the President should be protected by attorney-client privilege.

"Ultimately, if I were to say yes and he were to say no, I can't testify," Giuliani told CNN Friday . Not long after, news broke that he had told Sky News essentially the opposite that he would in fact testify.

Asked if he was concerned he would be subpoenaed by the House, Giuliani laughed.

"I consider them a joke. A sad joke. They have no legitimacy. I would think of challenging their subpoena on the grounds that they're not a legitimate committee," Giuliani said.

Through it all, Giuliani has put up a defiant front. "What they should be doing is giving me an award for uncovering something that when it's looked back on, someone's going to say, 'Why were you all asleep?'" he told CNN Thursday.

Rudy goes rogue

Rudy Giuliani finds himself at the heart of the events that led to President Trump's impeachment battle

Giuliani's role in Ukraine can be traced back to November 2018, when he was contacted by someone he describes as a "well-known investigator" who connected him with a Ukrainian-American businessman in Florida named Lev Parnas.

Through Parnas, who did not respond to CNN's request for comment, Giuliani began speaking with multiple Ukrainian nationals about information about two subjects of interest to his client, President Trump.

The first regarded what Giuliani has described as collusion between Democrats and Ukraine during the 2016 election. The second is the claim that Vice President Biden had pressured Ukraine to halt an investigation connected to his own son.

One of the people Giuliani says he spoke with in December 2018, over Skype, was Viktor Shokin, Ukraine's former prosecutor general. In 2016, Biden was among multiple Western leaders who successfully urged Ukraine to dismiss Shokin from the country's top prosecutor position, citing his insufficient work to root out corruption.

Viktor Shokin in 2015

But Shokin told Giuliani a different story: that he was pushed out to stop an investigation into Burisma Holdings, a Ukrainian natural gas company that included Hunter Biden as a compensated board member.

That story is littered with holes, and neither Giuliani nor anyone else has provided any evidence of actual wrongdoing by Biden or his son.

"Not one single credible outlet has given any credibility to his assertion," Biden said Friday. "Not one single one, and so I have no comment except the President should start to be president."

There is no evidence Hunter Biden was ever under investigation.

According to a report from Bloomberg, the Ukrainian government's case against Burisma had been "dormant" since 2014, two years before Shokin was dismissed at Biden's urging. Furthermore, Biden was also joined in his anti-corruption push against the prosecutor by numerous leaders in Europe as well as the International Monetary Fund -- none of whom had any family ties to Burisma.

Giuliani kept pursuing both stories, the Biden allegations and the so-called collusion in the 2016 election. Giuliani says he twice met with Shokin's successor as prosecutor general, Yuriy Lutsenko -- in New York in January 2019, then in Warsaw in February. Giuliani told CNN he learned more about both cases from Lutsenko, and that he also heard about Lutsenko's dissatisfaction with the US ambassador's supposed request that he drop certain prosecutions. In an April interview with the Babel, a Ukrainian news outlet, Lutsenko confirmed this account of their conversations.

Yuriy Lutsenko speaks during a briefing at the Central Election Commission in Kiev, Ukraine earlier this year.

According to the whistleblower's complaint, Giuliani's interactions with Ukrainian officials prompted concern among US officials about the "circumvention of national security decision-making processes."

Indeed, Giuliani's actions alarmed a number of career State Department officials , prompting some of them to keep distance between their work and his, according to multiple sources familiar with the matter. Specifically, they worried that being involved with his efforts would mean they were acting with a partisan political agenda, which as career foreign service officers they were not permitted to do.

Giuliani told CNN this week he never heard about any of those concerns. "If they were going to get upset about this, they should have gotten upset about it in January or February," he said.

But there were limits to Giuliani's freelance diplomacy, and it turns out Lutsenko was hardly a reliable source of information. That's partly due to the fact that Lutsenko's hold on power was precarious following Ukraine's April presidential election. Lutsenko may also have had ulterior motives for pushing the Biden conspiracy with Giuliani. The transition between presidents in Ukraine meant that his job as prosecutor general was in jeopardy, and having a line to a close adviser to the US President was a good way to stay relevant.

Indeed, Lutsenko would end up backing off some of his claims that Giuliani worked to amplify, including that his office was reopening its investigation into Burisma and Biden. Lutsenko would leave his position as prosecutor general on August 29.

The administration gets involved

JUST WATCHED Cuomo presses Giuliani on Ukraine: Give me the proof Replay More Videos ... MUST WATCH Cuomo presses Giuliani on Ukraine: Give me the proof 01:22

The public first got wind of Giuliani's Ukraine actions in May, when press reports, including one in the New York Times, described how Giuliani was promoting allegations about Biden and Burisma.

At the time, Giuliani told reporters, including CNN, that he was planning to travel to Ukraine to gather more information. Within a day of reports of his trip however, Giuliani canceled, telling CNN he was concerned that President-elect Zelensky had advisers around him who were "very vocal opponents of President Trump and peculiarly vocal supporters of Hillary Clinton" and that his trip would have accomplished little.

From there, the story faded from public view, but Giuliani kept working behind the scenes. A crucial development came on July 19, when Giuliani says the State Department's special envoy to Ukraine, Kurt Volker, offered to place him in touch with one of Zelensky's top advisers, Andriy Yermak.

At about the same time, Trump asked his acting chief of staff Mick Mulvaney to put a hold on millions in military aid to Ukraine. Trump has denied his decision to delay the aid had anything to do with his desire for Ukraine to investigate Biden.

Over the next two weeks, Giuliani told CNN he spoke with Yermak at least two times on the phone. He also said he reported back to Volker and the State Department on his conversations with Yermak.

According to the whistleblower complaint, "multiple officials" told the whistleblower that Ukrainian officials were "led to believe that a meeting or phone call between" Trump and Zelensky would depend on Zelensky's willingness to "play ball" on the cases raised by Giuliani.

Several days after the July 25 call, Giuliani told CNN he met with Yermak in person in Madrid on August 1.

After their meeting, Giuliani told CNN that he called Volker to relay the message he'd gotten from Yermak— that Zelensky was ridding himself of "leftovers and bad actors."

"I relayed back to him that I thought the guy was straight," Giuliani told CNN.

The whistleblower's complaint claims US officials characterized the August Madrid meeting as a "direct follow-up" to Trump's call with Zelensky.

Giuliani told CNN he was not aware that his August 1 meeting with Yermak was intended to follow up on Trump's call with Zelensky. Giuliani claimed he was unaware that Zelensky and Trump had brought his name up. He also said he never spoke with Trump about his meetings with Yermak. Giuliani claimed to CNN that he was not aware that he had been brought up in the July 25 phone call.

A wild week

By Tuesday night, Giuliani was texting and calling with CNN, mocking House speaker Nancy Pelosi's announcement about the impeachment inquiry. Minutes later, he was on Fox News , echoing his claim to CNN that he had a phone full of text messages proving that he was acting at the behest of US officials.

"I never talked to a Ukrainian official until the State Department called me and asked me to do it," he told Laura Ingraham Tuesday.

The next morning, the White House released the transcript of Trump's July 25 call with Zelensky, and Giuliani was relatively quiet and unresponsive to text messages from CNN.

But on Thursday, however, things ramped up again. The whistleblower complaint was released and the acting Director of National Intelligence was at a hearing in front the House Intelligence Committee. And it put Giuliani in full-attack mode.

Included in the complaint was the whistleblower's claim that two State Department officials had spoken to Giuliani to "contain the damage" he was doing to US national security interests regarding his work with Ukraine -- something Giuliani strongly disputed.

"At no time did either one of them say they wanted to contain damage," Giuliani told CNN . "At no time did the State Department in communication with me ever relay any of that information you're talking about."

Again, Giuliani went out of his way on the phone to CNN to point out that he had been in touch with Volker, telling CNN he had a "nice little trail" of text message conversations with Volker that would prove the State Department was fully aware of his actions regarding Ukraine.

But by now, the damage was done. Giuliani's involvement was enough not only to help prompt the House's impeachment inquiry but to raise the possibility he could be a witness.

Kurt Volker, United States Special Representative for Ukraine Negotiations, resigned on Friday

Events began to move very quickly by Friday afternoon. News first broke that House Democrats had subpoenaed Pompeo. A few hours later, Volker resigned from his post as the top diplomat to Ukraine. Giuliani's defense of himself -- that his Ukraine work had the blessing of the State Department -- was already causing heads to roll.

The 75-year-old former mayor, speaking with CNN Friday, was practically sanguine about this last development.

"He seemed like a very knowledgeable guy to me," said Giuliani, when asked about Volker's departure. "And a decent guy."