An adviser to the United States government, who spoke on the condition of anonymity because he was not authorized to talk to the press, agreed. “Iran’s preference was not to get engaged because the flow of events was in their direction,” he said. “If the Saudi intervention changes the calculus, they will be more aggressive.”

Though Bahrain said it had invited the force, the Saudi presence highlights the degree to which the kingdom has become concerned over Iran’s growing regional influence, and demonstrates that the Saudi monarchy has drawn the line at its back door. Oil-rich Saudi Arabia, a close ally of Washington, has traditionally preferred to operate in the shadows through checkbook diplomacy. It has long provided an economic lifeline to Bahrain.

But it now finds itself largely standing alone to face Iran since its most important ally in that fight, Hosni Mubarak of Egypt, has been ousted in a popular uprising. Iran’s ally, Hezbollah, recently toppled the Saudi-backed government of Lebanon — a symbol of its regional might and Saudi Arabia’s diminishing clout.

But Bahrain is right at Saudi Arabia’s eastern border, where the kingdoms are connected by a causeway.

The Gulf Cooperation Council was clearly alarmed at the prospect of a Shiite political victory in Bahrain, fearing that it would inspire restive Shiite populations in Saudi Arabia and Kuwait to protest as well. The majority of the population in Saudi Arabia’s eastern provinces, where the oil is found, is Shiite, and there have already been small protests there.

“If the opposition in Bahrain wins, then Saudi loses,” said Mustafa el-Labbad, director of Al Sharq Center for Regional and Strategic Studies in Cairo. “In this regional context, the decision to move troops into Bahrain is not to help the monarchy of Bahrain, but to help Saudi Arabia itself .”

The Bahrain government said that it had invited the force in to help restore and preserve public order. The United States — which has continued to back the monarchy — said Monday that the move was not an occupation. The United States has long been allied with Bahrain’s royal family and has based the Navy’s Fifth Fleet in Bahrain for many years.