By Ryan Browne and Barbara Starr, CNN

(CNN) -- An ardent opponent of the US military's presence in Afghanistan who once called for the use of lethal force against illegal immigrants and has made a litany of racist comments has been made a senior adviser at the Pentagon.

A Pentagon spokesman confirmed Wednesday that retired Army Col. Douglas Macgregor "will be serving as a Senior Advisor to the Acting Secretary of Defense. Mr. MacGregor's decades of military experience will be used to assist in the continued implementation of the President's national security priorities."

Macgregor's appointment is the latest in a series of sweeping changes at the Pentagon that has put defense officials on edge and fueled a growing sense of alarm among military and civilian officials, who are concerned about what could come next.

Axios was first to report Macgregor's appointment.

Four senior Pentagon officials have been fired or have resigned since Monday, including Defense Secretary Mark Esper who was fired in a tweet by President Donald Trump, Esper's chief of staff and the top officials overseeing policy and intelligence.

The moves will likely only add to the sense of chaos within the Pentagon following Trump's firing of Esper, which came two days after his Democratic opponent, Joe Biden, was projected as the winner of the presidential election -- a conclusion that Trump has refused to accept.

Esper was replaced by Christopher Miller, the director of the National Counterterrorism Center.

Those who assumed new roles included controversial retired Brig. Gen. Anthony Tata, who moved into the Pentagon's top policy role, taking over the duties of James Anderson, who resigned Tuesday, according to another US defense official.

Macgregor has been a vocal opponent of the US military's mission in Afghanistan and has called for a total withdrawal of US troops and the American Embassy despite the continued presence of terrorist groups there.

Knowledgeable sources told CNN's Jake Tapper on Tuesday that the White House-directed purge at the Defense Department may have been motivated by the fact Esper and his team were pushing back on a premature withdrawal from Afghanistan that would be carried out before the required conditions on the ground were met, and other pending security issues.

US military officials have long stressed that the US withdrawal from Afghanistan is conditions based, with those conditions including the Taliban's breaking its ties to al Qaeda and making progress in peace talks with the Afghan government, two conditions that have yet to be met.

But despite the lack of progress, the Trump administration has already substantially reduced US troops in the country by more than 50%, bringing the number of US military personnel there down to about 4,500, the lowest levels since the earliest days of the post 9/11 campaign.

Trump's national security adviser, Robert O'Brien, has advocated for a more accelerated withdrawal from Afghanistan irrespective of conditions on the ground, something made more feasible by the installation of White House loyalists in senior defense posts.

He has also called for an immediate end to the US military effort in Syria, where a small number of US troops back the Kurdish-led Syrian Democratic Forces in their fight against ISIS.

Macgregor advocated for the use of lethal force against unarmed migrants

Macgregor once advocated for the use of lethal force against unarmed migrants to deter illegal immigration from Mexico and Central America.

He was nominated by Trump this summer to be the ambassador to Germany but faced fierce opposition for his remarks uncovered by CNN's KFILE.

KFILE reviewed dozens of radio and television interviews with Macgregor and found he often demonized immigrants and refugees. He warned Mexican cartels were "driving millions of Mexicans with no education, no skills and the wrong culture into the United States, placing them essentially as wards of the American people." He repeatedly advocated instituting martial law at the US-Mexico border and to "shoot people" if necessary.

He also said that Eastern Ukrainians are "Russians" on the Russian state-controlled TV network RT in 2014 after Russia tried to annex Crimea and began an ongoing war with Ukraine over the territory -- positions not supported by the European Union and United States. He lamented that the US government intervened against Serbian forces, who engaged in ethnic cleansing and war crimes, during the Kosovo War in the 1990s to "put, essentially, a Muslim drug mafia in charge of that country."

This story has been updated with additional developments Wednesday.