Under Republican and Democratic Presidents alike, the U.S. Military was built into, possibly, the finest force, in history, to basically, prevent a Blitzkrieg like invasion from an overrunning our allies, to allow time for them to mobilize.
The invasion of Iraq, under George W. Bush, changed the purpose of that force. Shock and awe, designed to stop an invading force, was, instead used to actually invade and occupy a country, and, in effect, took a finely honed sword and used it as a meat cleaver.
We may never know the full extent of the damage that had been done to our armed forces, but, nevertheless, damage has been done. Obama inherited a wounded military that was unlike the military that George W. Bush had inherited from his father and Clinton.
Obama's reluctance to putting boots on the ground in Syria may stem more from his judgement as to the need for time to regroup than any thing else.