But he agrees with Gadson's assessment of the flexibility of the modern fighting force, born almost 40 years ago in the wake of Selective Service System drafts during the Vietnam War.
"We could never have imagined that it would be an all-volunteer force that would continue to operate through two extended protracted wars," he says. "Not only has it continued to function—we never had to call on selective service—but other than that, we arguably have the finest, best trained, most combat experienced military we've ever had."
"That's an extraordinary success story, but Greg is right. It's fragile," Nagl adds.
An ability to understand and train foreign enemies and allies has been at the forefront of U.S. efforts in Iraq, says Nagl, and those efforts contain the most valuable lessons he hopes the military does not forget.
Some of the most brutal conflicts in Nagl's Iraq tours took place in Al Anbar Province, where he says he would have traded a platoon of infantry for two good interpreters that he could trust.
"They were more valuable than riflemen," he says. "There was never a time in Iraq where we couldn't go anywhere we wanted to go, but we couldn't do what we needed to do because we didn't have the knowledge or support of the people who were there at that road crossing, in that mosque."
Now America hopes to draw down in Afghanistan in 2014, just as it pulled out of Iraq, yet the U.S. military has still not perfected training its allies to fight small wars abroad, Nagl says. Resurgent violence in Iraq may serve as a harbinger of what the U.S. will face in Afghanistan.
Al-Qaida conducted more suicide and vehicle bombings in 2012 than in 2011, according to a threat assessment report released March 12 from the Director of National Intelligence. These attacks are against almost exclusively Iraqi targets, though the fighters do not have sufficient strength to defeat Iraqi Security forces, the report states.
"That is the glaring failure of both Iraq and Afghanistan of the American military," he says. "We haven't figured out how to do that right, yet."