ISIS, Syria, and where Loyalties and Islam Factor in

ISIS is Sunni and they hate the Shiites
The Iraq government is mostly Shiite.
The government in Iran is Shiite.
The Assad government in Syria is fighting both ISIS and the Free Syrian Army.
ISIS is fighting the Free Syrian Army and Assad.
The Free Syrian Army is fighting Assad and ISIS.
The Free Syrian Army is not really an army, but hundreds of militias with varying loyalties and leaders, consisting of a myriad group of nationalists, radical Islamists, and some common Syrian folk with no other motivation than to live in freedom.
ISIS consists of the most ruthless of Islamic Sunni terrorists
The Shiites in Iraq would want us to kill ISIS because ISIS is Sunni.
The government in Iraq would want us to kill ISIS because it threatens the country's existence.
Many Sunnis in Iraq do not want us to kill ISIS, no matter how bad they are, because ISIS is also Sunni.
Assad, the bad-guy dictator in Iraq has no religion other than his own power.
Syria is about 75% Sunni so they would hate the Iraq government and Iran which would align them with ISIS, but ISIS is fighting Syria and the Free Syrian Army.
Governments, even Sunni governments see ISIS as a threat to their existence as ISIS wants to establish a caliphate, fundamental Islamic state from sovereign territories.

In the year 682 AD, Muhammad died and left no clear succession wishes.  In an argument over how leaders or caliphs, should be assigned, the Sunni and Shia split began.  This led to a series of tribal wars.  The Shia and the Sunni have been killing each other ever since with short periods of cooperation to work for Islam World domination.

If this seems too simple so far, the Shia and Sunni are subdivided into tribes which also fight each other over long forgotten and steel trapped remembered grievances.

It's a bloody fight over dogma.  In today's American culture, it would be similar to the Presbyterians having periods of wars with the Lutherans over cut bread or wafers, with short periods of cooperation for Christian World domination.

The Free Syrian Army and the rest of the rebels in Syria are no match in a fight with ISIS.  The rebels' mission is to take over the Syrian government and depose Assad and ISIS is an annoyance that is getting in the way of that mission.  The rebels are ill-equipped, lack training, funding and are not fighting for God.  The US has been training them and can get them all the equipment they need to win, but we cannot give them the united motivation that fighting for God can give ISIS.  Besides, they would rather somebody else take care of ISIS so they can get back to taking over the Syrian government.

Syrian rebels and Iraqi soldiers will fail as our air-supported cannon fodder, and ISIS will get a new batch of American ordnance and ammo.