Though President Donald Trump ordered the withdrawal of U.S. troops from Syria, the next mission in the country has already begun. As troops have left northern Syria over the past few weeks, new soldiers have begun moving in, heading to a remote base in the eastern part of the country.

On the president's orders, many U.S. special operations forces who were targeting the Islamic State militant group — like the ones who executed the raid that killed ISIS leader Abu Bakr al-Baghdadi in late October — are leaving the country. Replacing them are infantry troops, including National Guard soldiers tasked with helping the Syrian Defense Forces (SDF) hold territory. NBC News got an exclusive on-the-ground look at their deployment.

In the eastern region, that territory includes vast, strategically important oil resources. Though the footprint of troops on the ground has changed, the overall mission remains the same, U.S. officialsin Syria told NBC News.

"The enduring defeat of ISIS mission that we have, the oilfields are contained inside of that," Lt. Gen. Pat White said. "It denies them revenue, denies them an opportunity to reconstitute."

Trump's accelerated withdrawal plan, which he announced in mid-October, ended most of the U.S. military's roughly 1,000-person presence in the country, leaving only about 300 troops in the Al Tanf base in the south. According to Defense Secretary Mark Esper, most of the U.S. troops that pulled out of northern Syria in the past month are heading to western Iraq to continue the campaign against ISIS militants.

The new troops in the east are in addition to the 300 who remained. They are there to support the SDF, a longtime U.S. ally that has held more than 10,000 ISIS members in detention centers and prison camps, though some have escaped as U.S. troops have pulled out.

NBC News Correspondent Courtney Kube and Major General Eric Hill in a helicopter over eastern Syria. Nightly News

"We are still partnered with the Syrian Democratic Forces," said Maj. Gen. Eric Hill, commander of U.S. operations in Iraq and Syria. "All of the infrastructure that's in those areas will be secured by the Syrian Democratic Forces."

The partnership includes training, equipping and advising the SDF troops on how to operate to counter ISIS militants with less U.S. support, Hill said. But what's still unclear is how many U.S. troops will be in Syria once the withdrawal from the north and the west is over and the new deployments into Syria are complete.