A Saudi-led military force has crossed into Bahrain to prop up the monarchy in the tiny island nation against widening demonstrations, launching the first cross-border military operation to quell unrest since the Arab world's rebellions began in December.
Opposition groups immediately denounced Monday's intervention as an occupation that pushed Bahrain dangerously close to a state of "undeclared war."
Bahrain's majority Shiite Muslims see an opportunity to rid themselves of two centuries of rule by a Sunni monarchy. But Gulf Sunni leaders worry that any cracks in Bahrain's ruling system could threaten their own foundations. Protests are already flaring in Oman, Kuwait and even tightly ruled Saudi Arabia.
Gulf leaders are also concerned that political gains by Bahrain's Shiites might give Shiite Iran a stepping stone to its arch-rival Saudi Arabia, connected to Bahrain by a wide causeway.
Instead, the Saudis and other members of the Gulf Co-operation Council (GCC) sent forces the other way, deploying about 1,000 troops by land and air and cementing the entire six-nation alliance to the fate of Bahrain's rulers, who are key US allies as hosts of the American Navy's 5th Fleet.
Shortly after word of the foreign military reinforcements began spreading through Bahrain, protesters blocked roads in the capital Manama. Thousands of others swarmed into Pearl Square, the symbolic centre of the month-long revolt.
The worries about Iran have persisted, even though there are no apparent links between Tehran and Bahrain's opposition.
"The Gulf leaders have tried to legitimise this. They portray it not as intervention in an internal Bahrain dispute, but rather as an action against an external threat," said Sami Alfaraj, director of the Kuwait Centre for Strategic Studies.
In Tehran, authorities had no comment on the Gulf force moving into Bahrain. Iranian Foreign Minister Ali Akbar Salehi called on Bahrain to avoid using "violence and force", according to the semi-official Fars News Agency.
In Washington, White House spokesman Jay Carney pointedly did not call on the Saudi-led force to withdraw. "We are calling on the countries in the region to show restraint and pointing to the fact that the dialogue that can bring about political reform is essential for the stability of the countries in the region and their continued economic prosperity," Mr Carney said.