For almost a year and a half, John Bolton was among the most powerful government officials on Earth. A detail-oriented bureaucratic operator, Bolton got to push his hard-line views on issues such as Iran and North Korea on President Trump, the easily distracted political novice.

But ironically, it turns out Bolton may be more powerful outside the White House than in it. A little over a month ago, Bolton was fired as White House national security adviser; now, he is poised to play a central role in a scandal that embroiled Trump and his allies, including Secretary of State Mike Pompeo and personal lawyer Rudolph W. Giuliani.

Fiona Hill, formerly the White House’s top adviser on Russia, this week told House investigators looking into the impeachment of Trump that Giuliani ran a shadow foreign policy in Ukraine designed to personally benefit the president. According to those with knowledge of her testimony, who spoke on the condition of anonymity to disclose details of her deposition, Hill recounted a number of moments when Bolton, her direct superior at the White House, had angrily objected.

The national security adviser was furious about Giuliani’s pressure on Ukraine to investigate Trump’s political rivals, telling others that the former New York mayor was a “hand grenade who’s going to blow everybody up.” Bolton had also gone “ballistic” after one meeting that involved Giuliani, Hill said, according to an official who gave details of the testimony.

There is now speculation about whether Bolton could be called to testify to impeachment investigators and if he were to talk, what he might reveal. He has no reason to hold back: Bolton left the office on bad terms — publicly disputing whether he was fired at all — and he has already privately criticized Trump.

Bolton can be both calculated and combative. He is not a cautious career wonk like Hill or a diplomat like Michael McKinley, the former senior adviser to Pompeo who testified Wednesday, but a former Fox News pundit: He once wrote that being called “human scum” by North Korea was “the highest accolade I received.”

But while Bolton’s revenge may be vindicating, it will also be bittersweet. Any standoff with the White House will add to the idea that despite his fierce reputation, rigid worldview and lengthy résumé, his legacy in the Trump administration was easily overshadowed by an impulsive president’s instincts and whims.

He was forced to watch the president fawn over North Korean leader Kim Jong Un after months of missile tests. As diplomatic outreach remains stalled, North Korea’s state media ominously announced Wednesday that Kim, riding a white horse on a sacred mountain, planned “a great operation to strike the world with wonder again.”

And though Bolton helped convince Trump to leave the 2015 nuclear agreement with Iran and other world powers, economic pressure on Tehran has only resulted in an escalation of military tensions that Trump seems ill-equipped for. According to reports, Bolton’s ouster came after he disagreed with a proposal to ease sanctions on Iran to facilitate a meeting.

There were other clashes over policy concerning Venezuela and Afghanistan, where Trump began to ignore his own adviser’s advice. Bolton’s isolation on policy issues is also evident on Ukraine, where it turns out senior figures such as White House acting chief of staff Mick Mulvaney and diplomats such as Gordon Sondland, U.S. ambassador to the European Union, were circumventing him.

Trump’s announcement last week that he intended to pull U.S. troops out of northeast Syria was another blow to Bolton’s legacy.

The move ceded control of the area to Turkey and Syrian forces loyal to Bashar al-Assad, while leaving the Syrian Kurds who had allied with the United States to fight the Islamic State. It contradicted promises Bolton had made in January, when he said the United States would need to ensure that Kurdish groups “who have fought with us in Syria” would not be put in “jeopardy” by any withdrawal of U.S. troops from the country.

The national security adviser earned personal condemnation from Turkish President Recep Tay­yip Erdogan for the remarks.

“The message that Bolton gave in Israel is unacceptable. It is not possible for us to swallow,” Erdogan said during a televised address to lawmakers in his political party, as he refused to meet with Bolton during his visit to Ankara.

Even for critics of Trump, the idea of Bolton as a whistleblower is unpalatable. His reputation as a hawk and history of calling for military strikes against countries such as Iran and North Korea have left many feeling uneasy at the portrayal of him as righteous.

“I mean, I knew John Bolton believed in regime change, but I always assumed he meant overseas,” Charles P. Pierce wrote sardonically in Esquire.

Though Bolton didn’t leave a list of achievements from his time in the White House, he did pursue his own ambitions. He was given free rein to follow his unilateral obsessions, attacking agencies in the United Nations and the International Criminal Court, and sidelining the traditional multiagency foreign policy process.

And when it came to issues such as Syria, Iran and North Korea, Bolton’s greatest impact was in what he stopped from happening, be it pulling the United States out of the Iran deal or, according to many reports, scuppering talks with North Korea in Hanoi.

When Trump first announced he would pull U.S. troops out of Syria late last year, it was Bolton and others who rushed to scale back the announcement. Bolton’s help blocking that withdrawal, rather than organizing a pullout that would address both Turkish and Kurdish concerns, contributed to the chaotic pullback last week.

Brett McGurk, former special envoy to the coalition against the Islamic State, told the Daily Beast this week that Bolton had insisted the United States would stay in Syria until Iran left and that U.S. officials blocked attempts by Kurdish forces to plan for an American exit.

For Bolton, getting revenge on Trump after his unceremonious exit from the president’s orbit may prove to be deeply satisfying. But for many others, his time inside government and outside still leaves a bad taste in the mouth.

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