WASHINGTON — With just days left as commander in chief, President Obama is confronting a wrenching decision on whether to move ahead with plans to arm Syrian Kurdish fighters battling the Islamic State in order to launch the long-awaited assault to retake Raqqa, the terrorist group’s de facto capital.

The choice before Mr. Obama has been a stark one.

One option would be forging a closer military alliance with the Syrian Kurds to maintain the momentum in the fight against the Islamic State, even though Turkey has denounced the Kurdish fighters as terrorists.

The other would be for Mr. Obama to leave the decision to the incoming Trump administration. Such a move could delay the Raqqa operation for many months and would mean that Mr. Obama would leave office without a clear path forward for seizing the most important Islamic State stronghold and its base for plotting terrorist operations against the West.

Mr. Obama convened a meeting on Tuesday of the National Security Council, which discussed the question, one of the most momentous of the United States’ campaign against the Islamic State, also known as ISIS or ISIL.