The entry of Saudi and other Gulf forces into protest-hit Bahrain pushes an issue of local unrest towards a regional crisis, but is unlikely to cow protesters and may even galvanise them, analysts say.
Demonstrations calling for democratic reforms in Bahrain, a Shiite-majority Gulf state that has been ruled by the Sunni Al-Khalifa dynasty for more than 200 years, began on February 14 and have escalated since.
Armed forces from fellow Gulf Cooperation Council (GCC) states Saudi Arabia and the United Arab Emirates entered Bahrain on Monday to help the government there deal with the protesters -- a move slammed by Shiite Iran, which already has tense relations with some of its Arab neighbours on the Gulf.
On Wednesday Bahraini police firing shotguns and tear gas crushed the camp in Manama of the pro-democracy protest in an operation that left five people dead and sparked regional Shiite outrage.
"The deployment of Saudi and Emirati forces to Bahrain marks an escalation of a political crisis into a... confrontation with potential regional implications," said Chris Boucek, a fellow in the Carnegie Middle East Programme.
"It appears that there is the belief that a major show of force will suppress Bahraini protesters and stifle their demands for political reforms. This appears increasingly unlikely," he said.
Instead, Boucek said, the intervention could probably spur the demonstrators on.
"Rather then working to bolster the Bahraini royal family, the presence of Saudi and Emirati forces will likely have a galvanising effect on protesters," he said.
"I think the decision of the (Bahraini) king to invite the GCC forces... does pose new dilemmas and new dangers to the crisis in Bahrain, which itself shows no signs of ending," said Salman Shaikh, the director of the Brookings Doha Centre.
"We're on a path to escalation rather than having entered into a meaningful dialogue," he said of the situation in Bahrain.
"I think there is a real danger... that there will be further tensions rising between the Gulf states and Iran," Shaikh added.
Iran's defence minister on Wednesday said that in asking Gulf troops to help quell unrest, Bahrain had committed a "strategic and political" blunder that would cost it its "legitimacy."
Iran's President Mahmoud Ahmadinejad described the intervention as "foul and doomed."
In tit-for-tat moves, Bahrain recalled its ambassador to Tehran in protest at Iran's "blatant interference" in its affairs, and Iran pulled out its envoy "in protest at the mass killing of the people of Bahrain by its government."
While some Gulf Arab states, especially Qatar, maintain good relations with Iran, there is a decades-long history of tensions between Iran and some of its neighbours across the water.
Iraq invaded Iran in 1980, a year after the latter's Islamic revolution, and received substantial financial support from some Gulf Arab states, especially Saudi Arabia, during the eight-year conflict.
More recently, there has been significant concern in Saudi Arabia and other GCC states over what they see as Tehran's ambitions for regional influence, and over its nuclear programme, which various countries allege is military.
Iran has also in the past claimed Bahrain as part of its territory. It still controls Gulf islands claimed by the UAE.
"As far as Iran-GCC relations, they've always been strained," said Alanoud al-Sharekh, a senior fellow at the International Institute for Strategic Studies - Middle East.
"Relations definitely might worsen," she said, "but I don't think it will be something that would induce Iran to make an overt move."
There have not been reports of GCC forces being engaged in the fighting, but violence has increased significantly in Bahrain since the intervention, after several weeks of largely peaceful protests.
Analysts said that other Gulf states were clearly worried by the unrest in Bahrain, with Sharekh identifying calls from some Bahraini protesters for the fall of the monarchy as probably being a special cause of concern.
"I think there is a danger of regional spillover, not least to Saudi Arabia" which has already seen protests in its Shiite-majority east bordering Bahrain, said Shaikh.
And "the fact that some of the GCC countries have sent in forces with Saudi Arabia indicates the level of determination they have to draw a line in the sand here," he said.