Here’s an interesting chart from a 2010 Defense Business Board study that looked at how evenly U.S. troops were sharing the burdens of war. It’s relevant given the fact that the alleged Afghan shooter was on his fourth combat deployment.
Bottom line: only 11% of U.S. troops had deployed three or more times. Forty percent had never been deployed and 30% had been deployed only once.
While the data may be a little dated (and broader than just the Army), it’s relevant because the constant flow of new personnel into the military means that the imbalance detailed here probably hasn’t changed much.