Syria gets ultimatum from Turkey, not the USA.

Foreign Minister Ahmet Davutoğlu

Turkish Foreign Minister Ahmet Davutoğlu has issued a stark warning to Syrian authorities to immediately halt all military operations that result in civilian death or there will be nothing left to talk about the steps that would be taken.   Today's Zaman,  hat tip to Walter Russell Mead

 Turkey has been blocking arms shipments from Iran.

Compare US Secretary of State's words three days ago:
She notes there is a "crescendo of condemnation" from world powers. "It is clear Syria would be better off without him". "We urge those countries still [supporting Syria] to get on the right side of history". Clinton presses world to shut tap on Syrian oil

Update from Strategy Page:
"Turkey is threatening to invade to halt the Syrian government from killing its people. This has angered Iran, which is backing the Syrian government, with words and security personnel (advisors and death squads). While Iran has hailed all the other Arab uprisings this year, Syria has been declared a conspiracy by Israel, America and Arab tyrannies. If the Turks invade, the Syrian military is toast. Hated by its own people and long inferior to the Turks, the Syrian army would most likely fall apart".

Added:  "Whether or not Assad survives in power, Syria will be crippled as an Iranian ally".