When President Trump presides over his first meeting of the National Security Council, possibly this week, he is likely to confront a decision whose urgency is matched only by its complexity: whether to arm Syrian Kurdish fighters poised to liberate the Islamic State’s Syrian stronghold in Raqqa.

Taking back Raqqa, along with Mosul in Iraq — where Iraqi forces backed by the United States-led coalition have freed about half of the city — will effectively eliminate the Islamic State’s self-declared caliphate. The consequences for the Islamic State will be devastating. It will no longer control significant territory within which to train and harbor foreign fighters or exploit resources. It will lose the foundation of its most compelling narrative: the construction of an actual state.

As the noose around the Islamic State tightens, it has tried to adapt by plotting, inspiring or taking responsibility for indiscriminate attacks around the world: a nightclub in Orlando, Fla., a promenade in Nice, France, a cafe in Dhaka, Bangladesh, a square in Istanbul. Recruits are being told to stay home and attack there. Foreign fighters are being pushed out of Iraq and Syria, back to where they came from. The Islamic State directs its external operations from Raqqa. Hence, the urgency of Raqqa’s liberation.

The only fighters capable of seizing Raqqa belong to our most effective partner on the ground — the Syrian Democratic Forces, a mixture of Arabs and Kurds dominated by the People’s Protection Units, a Kurdish militia. The S.D.F. can succeed only if it is armed to overcome the Islamic State’s ferocious urban resistance of snipers, suicide bombers and improvised explosive devices.