That means the new total will likely exceed the 500 troops deployed to Syria back in January 2017, when Trump was sworn in.

“What it will mean as far as U.S. forces on the ground will likely be about 600 troops, would be my guess,” Jack Keane, a retired Army general who has informally advised the Trump administration on national security issues, said of the oil mission.

“I think it needs to be a few hundred guys, 300 or 400 at least,” said Dana Pittard, a retired Army major general who was the senior U.S. commander in Iraq during the first year of the fight against ISIS in 2014-2015. “That’s about what I would be comfortable with, depending on how many sites they’re securing.

“These oil facilities can be huge,” he added, noting that “you could have companies at multiple sites, and that would put you at around 600 personnel plus.”

That would push the U.S. troop level in Syria to close to the 1,000 that were in the country when Trump ordered his withdrawal earlier this month, largely reversing his much-touted pullout.

The U.S. forces in Deir ez-Zor will be based at two sites about 25 miles from each other, known as Conoco and Green Village, U.S. Central Command chief Gen. Frank McKenzie told reporters Wednesday. But he didn’t provide any other details about the troop presence or its size, saying only, “We’ll await further decisions of the U.S. government about how that plan is going to look in the long term.”

U.S. troop levels rose from 500 when Trump took office to an official Pentagon estimate of 2,000 by the end of 2017. They fell after the president first took the military by surprise with a full pullout order last December. National security leaders talked Trump down from that into a slower withdrawal that halved the force to 1,000 troops earlier this year.

“We’re out. But we are leaving soldiers to secure the oil,” Trump said Sunday after his speech announcing the death of ISIS leader Abu Bakr al-Baghdadi in a Delta Force raid. “I want to bring our soldiers back home, but I do want to secure the oil,” he added, summing up the conflicted stance that the Pentagon has capitalized on.

By the time of those remarks, the military had presented Trump with several options for the scope of the presence to stay behind in Deir ez-Zor.

“Options have been briefed to the president as far as the size, the composition, and what they’ll be doing,” a defense official said early Monday, but there’d been “no final determination. That’s still being shaped, the scope and size of the force.”

The Pentagon plans for there to be fewer troops in Syria than the 1,000 that were present at the time of Trump’s pullout order.

“My expectation is that it will be fewer than we had before," Defense Secretary Mark Esper said Monday, referring to units equipped with heavy armored vehicles that the U.S. hasn’t previously deployed to Syria.

The forces in Syria will be split between the oil mission in Deir ez-Zor and al-Tanf base in the southeast, Joint Chiefs of Staff Chairman Gen. Mark Milley also confirmed in the same briefing.

Pentagon officials have put the size of the force staying at al-Tanf in the southeast at about 200. That leaves a margin of fewer than 800 troops that the Pentagon could deploy to Deir ez-Zor without completely undoing the withdrawal.

“That basically gives you a battalion,” said a former senior military officer who has commanded troops in the region. A battalion is a unit of troops that can range in size from about 400 to 1,000, depending on the part of the Army it comes from.

The Pentagon’s troop-count math for Syria has often been fuzzy, however. Just a month before the Pentagon adjusted its troop count to 2,000 in December 2017, for example, it had announced the departure of a Marine artillery unit from the country, suggesting that the real number of troops on the ground had risen even higher than 2,000 during Trump’s first 10 months in office.

The largest part of the oil security force would likely be drawn from an Army armored brigade that is currently deployed to Kuwait as U.S. Central Command’s “theater reserve” force. Part of one of that brigade’s battalions had already been in northern Syria supporting the special operations forces there before the pullout order, albeit without its Bradley fighting vehicles and Abrams tanks.

A brigade from the 4th Infantry Division, from Fort Carson, Colo., is currently handing the Kuwait mission over to the North Carolina National Guard’s 30th Armored Brigade as part of a previously scheduled troop rotation. It wasn’t clear whether the troops who have already moved into Deir ez-Zor came from the outgoing brigade, the incoming one or a different unit.

But the underlying value of the presence there will have less to do with oil than with maintaining a relationship on the ground with the Syrian Democratic Force — mostly made up of Kurdish militias — that the United States appeared to be abandoning with the pullout, said the former senior military officer.

“Keeping the flag flying in that area is important to let the Kurds know that we’re not completely walking away from them,” he said, adding that Deir ez-Zor “is kind of the heartland” of the remaining Syrian ISIS remnants.

“That’s the place to have that enduring presence. The oil is secondary, tertiary, not even an important factor,” he added. “What is important is to be able to continue to conduct operations against the terrorist cells operating in that part of the Euphrates Valley.”

Keane, the retired Army general, noted that no matter how many troops are deployed, their presence on the ground will also serve as an anchor for U.S. air power, a key part of the U.S. military campaign against ISIS that has shown little sign of ending anytime soon.

Morning Defense The latest news in defense policy and politics. Sign Up Loading By signing up you agree to receive email newsletters or alerts from POLITICO. You can unsubscribe at any time. This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

That link to fighters, bombers, drones, and helicopter gunships if needed “is much more important than the numbers of U.S. troops," Keane said. "You don’t have to actually be standing on every oil field."

The troops and the air power will also act as a deterrent to any move into the area by Syrian government forces or their Russian allies, Esper acknowledged when asked this week if the force’s mission also includes that task.

“The short answer is yes, it presently does, because in that case we want to make sure the SDF does have access to the resources,” Esper said, referring the Kurdish-led Syrian Democratic Forces.

That’s likely the reason for putting heavy armored vehicles into the province, said the former senior military officer. “We don’t need armor against ISIS, but it will definitely help deter the Russians, the Syrian regime, the Turks from trying to get into that part of Syria,” he said.