Why does the U.S. intervene militarily in Iraq but not in Syria?

Since Islamic State fighters entered Iraq from Syria, they have given the world a revolting look at their ruthlessness.
They have left the severed heads of their vanquished on spikes. They have filled mass graves with the summarily executed. They have slaughtered and enslaved ethnic minority Yazidis.
When the Islamic State drove tens of thousands of them up mountains, where many died of thirst, it seemed the last straw.
U.S. President Barack Obama ordered airstrikes to beat them back.
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But the same cruelty has fumed next door in Syria -- along with the atrocities committed by forces loyal to President Bashar al-Assad against rebel fighters.
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While the world focused on the plight of the Yazidis in Iraq, 500 Syrian civilians died practically unnoticed.
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So, why is the United States intervening in Iraq, while avoiding Syria?
No nation protects Islamic State. Even al Qaeda is disgusted by its viciousness.
But international political entanglement awaits any power pondering action in Syria, due to loyalties to al-Assad.
As the United States and its allies have called for action against his regime, China and Russia have put their foot down in the UN Security Council. And Russia has kept al-Assad well-armed.
Although Russia rails against Muslim extremists, a U.S. intervention -- even only against Islamic State -- would meet with deep distrust in the Kremlin, especially after the relationship with Washington hit the dustbin over Ukraine.
Iran, a close al-Assad ally, would likely also be up in arms. Tehran is allegedly supplying Shia militia Hezbollah, which has crossed from Lebanon to shore up al-Assad's troops.

Syria's war is a checkerboard battle -- government troops and Hezbollah in a fight against Islamic State and other resistance groups, who also battle each other.
Obama doesn't see a strong partner in the politically fractured Free Syrian Army, because it's hardly a formidable force against al-Assad's professional army.
In Iraq, the battle lines are clearer -- the Iraqi government with U.S. support vs. Islamic State.
Baghdad has made a shaky showing against the militants. But in the north, Washington has a strong, loyal partner in semi-autonomous Kurdistan.
Many of the U.S. advisers deployed to Iraq are stationed in its capital Irbil to support Peshmerga troops in hot confrontation with Islamic state.

SOURCE:www.cnn.com