By Andrew North BBC News, Washington
What's the difference between Libya and Yemen or Bahrain?
All three states have been using violence to crush pro-democracy protests.
But only against Libya are the US and its Western allies planning a military response.
Yemen and Bahrain's crackdowns have so far been met only with words, not action.
On one level the answer is obvious.
Bahrain and Yemen are US allies - especially Bahrain with its large US naval base. Libya is not.
The US response to Bahrain is further complicated by neighbouring Saudi Arabia, Washington's number one Arab ally.
Sunni 'red line'
The Saudis were not happy to see Egyptian President Hosni Mubarak go.
Losing the Sunni monarchy in its neighbour is a red line - that's why it took the unprecedented step of sending 1,000 troops over the border into Bahrain, after which the crackdown began.
But what happened to the "universal values" US President Barack Obama cited when he eventually backed protesters in Egypt?
His decision to abandon an old US ally there - Mr Mubarak - gave some the impression he was preparing to apply those values universally and to break with the past US policy of cosying up to other Middle Eastern regimes.
Critics say it was a dangerous impression, raising protesters' expectations as well as Gulf monarchs' blood pressure.
'Interests come first'
"The US always preaches values that it cannot live up to," says Marina Ottaway, director of the Middle East programme at the Carnegie Endowment for International Peace in Washington.
"In the end, its interests come first."
As the uprisings have spread out of North Africa to Bahrain and Saudi Arabia, those interests have come to the fore again, with Washington taking a more cautious, country-by-country approach.
For the US, stability in those oil-rich states now appears to trump the hopes of their protest movements.
Its concerns about Iran play into this thinking. Most of the protesters are from Bahrain's Shia majority.
Both the Bahrainis and the Saudi authorities establishment have accused Tehran of helping to orchestrate the unrest. And some US officials fear the toppling of the Sunni monarchy would become a victory for Iran.
Yemen is crucial to Washington for its battle with al-Qaeda - which makes the Obama administration cautious in how hard it pushes Yemeni President Ali Abdullah Saleh.
"The US is very afraid that if Saleh goes, Yemen will fall apart," Ms Ottaway says.
Mr Obama condemned the latest violence in Yemen, in which at least 40 protesters were killed.
But he would only call for "those responsible... to be held accountable", without directly laying it at Mr Saleh's door.
Washington has had a low-key response as well to violence used by Iraqi security forces against protesters there.
Even with Libya, the new caution is on display. The administration was reluctant for some time to back a no-fly zone, fearing it could lead to a third US war on a Muslim country, after Afghanistan and Iraq.
It only did so only after it got support from Arab states and European allies.
And it is still not clear how much the US will contribute militarily to the UN-backed no-fly zone or what will happen if Col Gaddafi succeeds in hanging onto power.
With recent history in mind and the tide of protest still sweeping through the region, caution arguably looks a sensible policy from a US point of view.
But it also risks giving conservative Arab leaders the breathing space they need to stall the push for reform and hang on.
Having watched Tunisia and Egypt go, other Arab leaders are following Libya's lead in drawing a line in the sand and opting for force rather than dialogue.
It's not clear if Mr Obama can do anything about it.
BBC News - Why is US backing force in Libya but not Bahrain, Yemen?