Amidst a shake-up in the White House staff, President Donald Trump appointed John Bolton as his new national security adviser. Bolton is the third security adviser to serve under the Trump administration since Trump was elected.

Bolton’s appointment to the national security adviser position is a controversial one — in the past, Bolton has said that the U.S. should declare war on both North Korea and Iran, and he has been credibly accused of poorly treating his staff members.

The New York Times editorial staff wrote that “there are few people more likely than Mr. Bolton is to lead the country into war,” citing Bolton’s positions on relations with North Korea and Iran.

Sen. Lindsey Graham (R-S.C.), a retired U.S. Air Force colonel, has openly supported Bolton for the position. Graham told Fox News that Bolton is the perfect person to help Trump with negotiations with Kim Jong-Un.

“The North Koreans know, without a doubt, that John Bolton sees their nuclear program as a threat to the United States and will strongly advise the president to use military force if he has to, which means it’s less likely we will,” Graham said.

Bolton is stepping into the position to replace Lt. Gen. H.R. McMaster, the former adviser, who will officially resign on April 9, according to a tweet Trump sent on March 22.

“I am very thankful for the service of General H.R. McMaster who has done an outstanding job & will always remain my friend,” Trump said.

Bolton rose to public attention during the Bush administration, when he served as the U.S.’s ambassador to the United Nations. At the time of his confirmation hearing, TIME Magazine reported that Bolton had a reputation for “distorting vital intelligence and that Bolton “frequently tried to push the CIA to produce information to conform to — and confirm — his views.”

Critics of Bolton have cited his “hawkish” tendencies, combined with vacancies in crucial positions in the U.S. State Department, as cause for concern as the U.S. prepares for the upcoming summit meeting with North Korean officials.

Bolton’s appointment comes shortly after Trump fired Rex Tillerson, the former secretary of state, via Twitter on March 13.

“Mike Pompeo, Director of the CIA, will become our new Secretary of State,” Trump tweeted. “He will do a fantastic job! Thank you to Rex Tillerson for his service! Gina Haspel will become the new Director of the CIA, and the first woman so chosen. Congratulations to all!”

Trump tweeted out a message saying that Tillerson would be replaced at 5:44 a.m. on the morning of March 13, but Tillerson did not find out from Trump himself until noon.