The oil-rich monarchies of the Persian Gulf have relied for decades on the promise of protection by the United States military, a commitment sealed by the rollback of the 1990 invasion of Kuwait and reinforced by a half dozen American military bases that sprang up around the region.

Now that commitment is facing its most serious test since the first gulf war: an attack last Saturday by a swarm of at least 17 missiles and drones that crippled Saudi Arabia’s most critical oil installation and temporarily knocked out 5 percent of the world’s oil supply.

Washington and Riyadh blamed Iran, despite its denials, and President Trump threatened that the United States was “locked and loaded.” Yet despite months of such bravado, Mr. Trump has been hesitant to take military action that might risk an expanded conflagration. For better or worse, such a muted response could signal another turning point for the region.

“It is enormous,” said Gregory Gause, a scholar of the region at Texas A&M University. “This is the most serious challenge since the invasion of Kuwait to the status of the United States as a great power that would protect the free flow of energy from the region, and unless there is a big change in the response from the Trump administration I think Gulf leaders will start to question the value of that security commitment.”