First, the good news.  Despite fears that the unrest sweeping through the Arab world could destabilize Saudi Arabia and threaten oil supplies to the West, the Saudis have not yet seen the need to mobilize their security forces to protect their own government.  The bad news?  They’ve mobilized their security forces to protect Bahrain’s instead:

Saudi Arabian troops entered Bahrain on Monday as part of a military force from Gulf states called in to deal with a month of political unrest in the island kingdom.

Bahrain’s government called in forces from its Sunni neighbors to put down unrest after protesters overwhelmed police and blocked roads in a resurgence of mass protests seen last month.

Nabeel al-Hamer, a former information minister and adviser to the royal court, said on his Twitter feed these troops were already on the island, a key U.S. ally and headquarters of the U.S. Navy’s Fifth Fleet. Saudi officials declined comment.

Bahrain formally requested the assistance of the Gulf Cooperation Council, under whose aegis the Saudis landed in Bahrain today.  The troops will “maintain order and security,” which in this case means putting an end to weeks of demonstrations and unrest mainly from Bahrain’s Shi’ites, who by some estimates outnumber Sunnis 2-1 but who have little power in the constitutional monarchy.  Bahrain is one of the smallest nations in the world, with only 1.2 million people, with as many as half of those not native to the island but recent immigrants.  Despite its small population, its diversified economy makes it a relative powerhouse, although still dwarfed by the emirates in the region.

This will put a new wrinkle in the American reaction to the unrest.  Bahrain has a constitutional monarchy, as noted above, with a more liberal political environment than Saudi Arabia.  Both, however, are American allies; Bahrain has a free-trade agreement with the US.  Women have the right to vote and to seek education, which is much different than the Saudis.  The people have demonstrated peacefully for the most part in the Pearl Roundabout in the capital of Manama, but government forces used live ammunition to attempt to drive them out on at least two occasions last month.  They claim to want a republic based on representative democracy, exactly as protesters in Egypt, Libya, and Tunisia demanded — and which the US endorsed in those instances, to vacillating and varying degrees.

Now that one US ally has more or less invaded another, Grenada-style, at the request of a monarchy that has fired on its own people to maintain its power, what will Barack Obama do?  The Saudis clearly see the threat in Bahrain as a potential destabilizing force in their own country as well as fearing a growth of Shi’ite power in the region with the takeover of Bahrain.  Will Obama tell the Saudis to stand down and let the people of Bahrain settle their own accounts despite their probably-legitimate fears, or will he side with the Saudis for the status quo while the rest of the Arab world gets turned upside down?  Frankly, there aren’t a lot of great options here.