"I'm not going to pick on your words, but I would only — I would be cautious with saying that 'the mission [is] to secure the oil fields,'" said Rear Adm. William Byrne. "The mission is the defeat of ISIS. The securing of the oil fields is a subordinate task to that mission, and the purpose of that task is to deny ISIS the revenues from that oil infrastructure."

Trump this fall ordered the withdrawal of almost all of the 1,000 U.S. troops in Syria after learning that Erdoğan planned to send troops into the country's northeast to clear the area of YPG Kurds, which Turkey considers terrorists. Trump later switched course and decided to send in U.S. troops to protect oil fields in eastern Syria.

Joint Chiefs Chairman Gen. Mark Milley, speaking on ABC's "This Week" on Sunday, said the new mission means "maybe 600" troops will remain in Syria. He also emphasized that the fight against ISIS is the priority.

"There are still ISIS fighters in the region and unless pressure is maintained, unless attention is maintained on that group, then there's a very real possibility that conditions could be set for a reemergence of ISIS," Milley said. "So we’re committed to do that. The footprint will be small, but the objective will remain the same, the enduring defeat of ISIS."

Pentagon spokesperson Jonathan Hoffman, during the same press conference last week, said the revenue will stay with the Kurds.

"The revenue from this is not going to the U.S.," he said. "This is going to the [Syrian Democratic Forces]."