President Donald Trump accused Sen. Lindsey Graham on Thursday of being “against saving soldier lives [and] billions of [dollars]” as the two continue to feud over the president’s decision to declare victory over the Islamic State in Syria and pull out of the country.

Defense Secretary James Mattis told Graham on Thursday that he opposes the move and that ISIS has not been completely eradicated in Syria, despite the president’s victory proclamation on Wednesday.

“So hard to believe that Lindsey Graham would be against saving soldier lives [and] billions of [dollars],” Trump tweeted Thursday. “Why are we fighting for our enemy, Syria, by staying & killing ISIS for them, Russia, Iran & other locals? Time to focus on our Country [and] bring our youth back home where they belong!”

With his decision to pull U.S. resources from the military operation against ISIS in Syria, Trump appears to be disregarding what Democratic and Republican lawmakers have said are key strategic and national security concerns for the U.S. in the region.

By leaving Syria, Trump would cede a crucial sphere of influence in the Middle East to Russia and Iran; abandon Kurdish allies on the border between two hostile nations, Syria and Turkey, who do not recognize them or want them there; and leave the door open for ISIS to rebound from its military defeat, Graham and Sen. Bob Menendez, the top Democrat on the Foreign Relations Committee said.

“Christmas came early to Russia,” Menendez told reporters Thursday.

Graham said Thursday that leaving Syria now would be “akin to surrendering.”

“This will be seen by the entire world as a decision not based on sound military advice, but a decision based on frustration, and all of our allies are scratching their head right now,” Graham said. He said the British have been left scratching their heads with how to proceed if the U.S. ceases operations in Syria.

“In this war, you will not win it by giving up. This is an akin to surrendering,” Graham said.