One American expert in the Persian Gulf who advises policymakers in Washington said the Saudi king’s action was taken without regard for what might happen if it fails — if the violence leads only to more violence. The Saudi policy, he said, “is risky and could potentially draw us into conflicts we have not looked for.”

“What if the Bahrain venture fails, who will bail them out? It will have to be us.”

Saudi Arabia’s supporters acknowledge that this confrontation can escalate, but they tend to place the responsibility on Iran. “It can lead to that direct conflict if Iran were to interfere and use this as an excuse to interfere,” said Abdulaziz O. Sager, chairman of the Gulf Research Center, which is based in Dubai. “I hope Iran can understand that any interference will not be acceptable.”

There has been no evidence that Iran played a part in Bahrain’s uprising, which was led by young Bahrainis from the Shiite majority. Still, many protesters have said, it is reasonable to expect Shiites to be more receptive to Iran if they do gain power. There is little doubt, they also say, that a Shiite-led government would be less receptive to the Saudis.

Even some of the Iranian regime’s harshest critics are saying the Saudi military venture in Bahrain will change the narrative of the region in Iran’s favor. Abbas Milani, an Iranian who went into exile after the 1979 revolution and is now director of Iranian studies at Stanford University, put it this way: “Iran, as the most brutal authoritarian regime in the region, will now have the chance to seem to stand with the democratic aspirations of the people, and against authoritarians clinging to power.”

The Saudi king’s decision to back King Hamad bin Isa al-Khalifa’s crackdown in Bahrain also underscored the challenge the United States often faces with its closest allies in the Middle East, where some interests align — like protecting the flow of oil — and others do not, like financing global terrorism. Saudi Arabia has moved aggressively to cut off radical Islamic terrorism within its own borders, but it has addressed the global phenomenon with far less conviction, many American experts have said.