British infantry battalions are 17 percent under strength. About two-thirds of that is due to the usual causes (illness, attendance at military training courses, and vacation). But the rest is due to the inability of recruiters to attract enough young men (out of a population of 61 million) to serve in the infantry.
British units recruit locally, so the shortages reflect local recruiting problems. The two battalions with the biggest shortages are from the north. The 2nd Battalion (the Green Howards) of The Yorkshire Regiment is short 37 percent, while the 2nd Battalion (Royal Highland Fusiliers) of The Royal Regiment of Scotland is short 28 percent. Overall, the army is short two percent of its authorized strength. There's not a lot of problems recruiting for non-combat jobs.
In the U.S., with a population, and army, five times the size of Britain's, meeting recruiting goals has not been a problem, nor has adding additional troops to units headed overseas, so they depart near full strength. Most of this seeming success is due to different recruiting methods. Except in some reserve (National Guard) units, troops are recruited from all over the nation, for all units. Thus those parts of the country that produce a disproportionate number of recruits, help make up for the shortfalls in areas that don't have as many volunteers.
Being in the infantry is a tough job. While support troops worry about getting fat (obesity is a growing problem), the infantry have to be careful that they don't get injured during the strenuous training they constantly undergo. And then there are combat losses. In the past eight years, 232 British troops (mostly infantry) have died in Afghanistan, and nearly a thousand have been hospitalized for wounds, injury or diseases (there are lot of diseases in Afghanistan). But the biggest source of losses are troops that don't reenlist, or join in the first place.