BEIRUT, Lebanon — President Trump’s decision on Sunday to step aside and let Turkish forces come into northern Syria instantly cast into doubt the fate of ethnic Kurds there who have been the United States’ closest allies in the fight against the Islamic State, and who had worked to achieve a degree of self-rule in that stretch of Syria.

[Details on the sudden U.S. announcement and what it could mean for the region.]

Now, the question of who could provide a long-term deterrent to Iranian and Russian interests in the area — and help ensure that ISIS does not rebound in Syria — is suddenly very much in play again.