For years, the American strategic map of the Middle East was dominated by Iran at the center of power of a “Shia crescent,” with Syria as the funnel for the Iranian arms used by terrorism groups to attack Israel, and as the home to Russia’s naval and air presence in the region.

Yet when the Syrian government fell with astounding speed over the weekend after more than a half century of rule, shattering yet another critical element of the crescent, American intelligence officials were caught by surprise. As recently as Friday night, senior U.S. officials thought President Bashar al-Assad had a roughly even chance of holding on — even if that meant reaching for the chemical weapons he had used on his own people.

Washington awoke on Sunday morning to a new reality. It is perhaps the most momentous upheaval yet in the 14 months since Hamas’s Oct. 7, 2023, attack on Israel unleashed a wave of violent retaliation that changed the region’s power dynamics.

Now, with Mr. al-Assad’s ouster, two urgent and related questions are circulating through Washington, just six weeks before the inauguration of President-elect Donald J. Trump for his second term — one in which the world looks dramatically different than when he left office just shy of four years ago.

First, will the rebels evict the Iranians and the Russians from Syrian territory, as some of their leaders have threatened? Or, out of pragmatism, will they seek some kind of accommodation with the two powers that helped kill them in a long civil war?