Because of the war in Iraq and Afghanistan, the U.S. military needs a lot more blood. They won't say exactly how much more, for security reasons. But the Pentagon does admit that blood needs have increased four fold since 2001. On average, every million Americans uses about 40,000 units (usually unit is a half liter of whole blood) a year. Military personnel are younger and healthier than the general population, and probably require less blood in peacetime. But that implies that current military needs are over 100,000 units a year.
The military typically obtains all the blood it needs from it's own members and their immediate families. The many reasons for this are to save money (buying whole blood on the open markets costs about $250 a unit), to avoid causing any shortages in the civilian market and, because it's good for morale.
The morale angle is particularly important in wartime. While not everyone is in a combat zone, everyone knows someone who is, or will soon be. Giving blood is a very primordial way of supporting your fellow soldiers. This is especially true with family members.
While many troops, or family members, cannot give blood because of the new (post-AIDs/Mad Cow Disease/Etc) screening rules, there is still plenty donation capacity. One donor can give up to five units of whole blood a year. That means that maximum capacity of all active duty and reserve troops, and their immediate families, is way over ten million units a year. But with the increased demand, the call for blood is being given more often and, so far, supply has met demand.