Hagel worked behind the scenes but seemed overly deferential to generals in top-level strategy meetings, officials said.
The oil-rich United Arab Emirates is known by American generals for its ability and willingness to fight, but the full extent of the military cooperation has not been publicly acknowledged.
The focus on PTSD has distorted America’s perception of returning soldiers.
Officials do not believe the units will be able to seize towns from the Islamic State without the help of U.S. combat troops, which President Obama has ruled out.
The tiny Arab nation is offering itself as a U.S. ally on one hand, supporting Islamists with the other, and vexing neighbors in all directions.
The threat from the Islamic State has led Persian Gulf nations to put aside differences and join the U.S. effort.
The difficulty of battling the Islamic State in Syria will make the fight in Iraq “seem easy,” said a general.
More than half of the 2.6 million service members dispatched to fight in Iraq and Afghanistan say their health is now worse.
Authorities said the gunman was an Iraq war veteran with behavioral and mental health issues.
Allen said his decision was not influenced by a Pentagon investigation into e-mail messages he exchanged with Tampa socialite Jill Kelley.