It appears the Gulf Cooperation Council (GCC) has taken the unprecedented step of intervening militarily in the affairs of another member state to crush the democratisation movement there, which is led predominantly by Shia Muslims.
The units involved are from the GCC's Peninsula Shield force, which was established to combat foreign military aggression, not to police internal ructions in member states.
In effect, the al-Saud are moving to prop up Bahrain's al-Khalifa (also Sunni and hailing from the Arabian Peninsula) in a bid to prevent the Baharna (Shia) revolt crossing the causeway into Saudi - where sporadic demonstrations are already taking place in predominantly Shia, oil-rich Eastern Province.
This, and calls for martial law from Bahraini Sunni MPs, constitute a serious escalation and threaten a second Shia uprising in Bahrain in which the protest movement is transformed into an anti-occupation struggle. In fact, this is likely the intended effect as there was every indication that the NVDA of the protestors was building tremendous pressure on the government.
Sporadic sectarian clashes have broken out in recent days and weeks between Shia and Sunni Bahrainis and Shia and foreign Sunni residents - accused by Baharna of being part of a sectarian cleansing effort by the al-Khalifa wherein foreign Sunni are naturalised.
This intervention would not have been possible without US support and is made under the cover of the Japanese and Libyan crises. It appears to speak to Washington's acceptance of al-Khalifa/al-Saud opportunistic arguments that Iran is playing a role in the social unrest, of which there is no evidence.