Historically, the Roman Empire went into decline when its own citizens were no longer able to carry the Roman military burden and the task had to be given over to Germanic tribesmen, and in other news...

...Army rejects nearly 90% of American applicants for being obese or unable to run, jump or roll:

Retired Lt. Gen. Mark Hertling said he was floored by what he found in 2009 when he was assigned to overhaul the Army’s training system. Seventy-five percent of civilians who wanted to join the force were ineligible, he said. Obesity was the leading cause.
“Of the 25 percent that could join, what we found was 65 percent could not pass the [physical training] test on the first day,” he said in a recent speech. “Young people joining our service could not run, jump, tumble or roll — the kind of things you would expect soldiers to do if you’re in combat.”
 

As the Iraq and Afghanistan wars strained the military, the Army granted waivers to recruits who would normally not be eligible — for example, people who were overweight or who had criminal records. But now, under orders to reduce the active-duty force from 570,000 to 490,000 by 2017, the Army has ordered commanders to weed out substandard troops. “We will use the drawdown as an opportunity to shape our Army by ensuring that we retain only the very best soldiers,” Army Secretary John M. McHugh wrote in a Feb. 2 memo on retention initiatives.