Even options short of direct strikes pose difficulties.
Grounding Assad's air force by enforcing a no-fly zone in Syria would most likely require a large-scale attack on the military's advanced air defense systems. Military support for the opposition continues in the form of small weapons and ammunition. But proposals for sending more powerful weaponry raise fears that it could fall into the hands of extremist rebel groups, which are melding with moderate rebels.
The U.S. remains opposed to Saudi Arabian deliveries of shoulder-launched, anti-aircraft missiles because of the potential risk to commercial aircraft.
"Right now we don't think that there is a military solution," Obama said last week following talks on Syria with French President Francois Hollande. At the same time, Obama called the situation on the ground "horrendous" and acknowledged "enormous frustration" with peace talks in Geneva, which ended last weekend without progress.
By any account, the U.S. policy of sending limited military aid for Syria's moderate opposition coupled with support for U.N.-brokered peace talks between the rebels and Assad's government isn't working.
Appalling scenes of emaciated children leaving the besieged city of Homs last week underscored the desperate plight of many Syrians — and the potential for more suffering. A second round of Geneva negotiations ended last weekend with little promise for a future breakthrough and with fresh American frustrations with Russia, which is Assad's most powerful military and diplomatic supporter.
The administration concedes Assad's hold on power has strengthened.
On Tuesday, Secretary of State John Kerry offered lingering hope that peace talks could yield results.
However slowly, the potential for a terrorist base developing in northern Syria akin to Afghanistan before the Sept. 11, 2001, attacks may be changing the administration's thinking.
In recent weeks, the president's senior national security aides have delivered dire warnings about extremist havens in Syria and about Americans and other Westerners joining the fight and being radicalized.