“Arming the Kurdish elements of the S.D.F. will make them more militarily effective against ISIS in Raqqa,” said Andrew J. Tabler, an expert on Syria at the Washington Institute for Near East Policy. But, he added, referring to President Bashar al-Assad of Syria, “if they don’t expand to include more of the Sunni Arab tribes of the Euphrates River valley, who make up the majority there, the S.D.F. will have a hard time holding that area because of the Kurdish-Arab split, leaving that area vulnerable for an Assad regime comeback.”