Two former U.S. officials who led the global fight against the Islamic State are warning Americans about a new threat to the homeland: homegrown white nationalist terrorism.

Retired Marine Gen. John Allen and Brett McGurk, both of whom served as special presidential envoys for the global coalition taking on ISIS, said in a Washington Post op-ed that the word "terrorism" must be used to describe the new national security threats facing the country from white supremacist groups.

"The terrorist acts may differ from Islamic State attacks in degree, but they are similar in kind: driven by hateful narratives, dehumanization, the rationalization of violence and the glorification of murder, combined with ready access to recruits and weapons of war," they wrote Tuesday.

Their warning follows federal authorities' announcement that they're treating Saturday's mass shooting at a shopping center in El Paso, Texas, as a domestic terrorism case. The suspect in the attack, who allegedly posted a racist manifesto before the shooting, could also face hate-crime charges.

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Allen, who served for nearly four decades before retiring in 2013, commanded Marines in Iraq and oversaw the NATO-led mission in Afghanization. About a year later, President Barack Obama named him the special presidential envoy for the global coalition against ISIS. While he was in that position for 15 months, the coalition grew to 65 members.

Allen later endorsed Hillary Clinton for president during the 2016 campaign.

McGurk, a former diplomat who has served in key national security positions under Presidents George W. Bush, Obama and Donald Trump, took over for Allen in 2015. He remained in the position until December, when he resigned following Trump's sudden announcement that he wished to withdraw troops from Syria.

The U.S. must take a leadership role in overcoming white nationalist terrorism before it gets worse, Allen and McGurk wrote. Left unchecked, they say it could result in bigger attacks similar to the 1995 Oklahoma City bombing, which killed 168 people.

"The country now confronts a national security emergency on par with the Islamic State threat," they added, which requires leadership at every government level.

"It demands moral clarity and a call from the Oval Office directing all assets of the federal government to develop a comprehensive, long-term campaign to protect all Americans," the op-ed states. "If the president will not act, then Congress and state and local governments must instead.

"The matter is too urgent to wait for new national leadership -- at stake is nothing less than the protection of the American people and our way of life," it continues.

ISIS has largely been defeated thanks to a comprehensive plan involving all departments and agencies in the U.S. government to vanquish it on the battlefield, Allen and McGurk said. The same approach is now needed to defeat white nationalist terrorism, they say, which has claimed lives in attacks in El Paso, South Carolina, Norway and New Zealand.

Officials at the Department of Homeland Security have made attempts to combat domestic terror threats from white supremist groups, CNN reported Wednesday. But the Trump administration has sought to focus on jihadist threats, the outlet reported, citing unnamed sources.

Trump has downplayed the threat posed by white nationalist groups, even as some accuse him of fanning anti-immigrant sentiment among Americans. When a gunman was accused of killing 49 people at a New Zealand mosque in March, Trump expressed sympathy but said he saw no evidence that white nationalism was on the rise.

"I think it's a small group of people that have very, very serious problems," he said at the time.

On Wednesday, he said he's concerned about the rise of any hate group, including white supremacists or anti-fascist groups. He denied that his rhetoric has played a role in emboldening any of them.

"I think my rhetoric brings people together," the president said.

The military has shown some recent signs of racist and white supremacist behavior in the ranks.

Coast Guard Lt. Christopher Hasson, a self-described white supremacist, was arrested in April for allegedly plotting to kill Democratic politicians and famous broadcast journalists.

In May, Hawaii-based Marine Lance Cpl. Mason Mead was awaiting discharge after pleading guilty to advocating supremacist ideology when he was caught posting Nazi propaganda and other racist items online.

In January, a Coast Guard officer was punished for flashing a white supremacy hand signal on live TV while another service member was being interviewed about the military's response to the deadly Hurricane Florence.

And Air Force Staff Sgt. Geraldine Lovely was recently booted from the service for posting a profanity-laced rant about black female subordinates last year.

-- Gina Harkins can be reached at gina.harkins@military.com. Follow her on Twitter @ginaaharkins.