If you're a military service member serving abroad right now, staying in touch with your loved ones back home isn't all that difficult. Assuming you have access to some sort of broadband service or wireless network, you can send a text, write an email, or maybe even make a video call. But all of those are relatively new. From the summer of 1942 until the fall of 1945, as many as two million American soldiers deployed to Europe to fight Nazi Germany -- and they, too, wanted to stay close to those at home. 

The easy and obvious solution to this problem is an archaic form of communication called "writing a letter." (That's a joke, please don't yell at me.) Soldiers could -- and did -- find a pen and piece of paper, share their thoughts and fears with those back stateside, and drop them into a mailbox (or the equivalent). And their families and friends could do the same, just in the reverse direction.

And this caused a new problem. With so many Americans in Europe at the same time, the U.S. military was inundated with letters. Transporting millions of millions of notes from the U.S. to those stationed abroad was overwhelming; the planes bringing mail across the Atlantic were needed to bring food, arms, and other supplies as well. In other situations, the government put limits on stateside activities that hampered the war effort, and restricting letter-writing could have been a solution. But the powers that be didn't want to go that route; they saw the mail as critical to the war effort. The National World War II Museum explains:

The critical nature of the mail effort was addressed in the 1942 Annual Report to the Postmaster General which stated: “The Post Office, War and Navy departments realize fully that frequent and rapid communication with parents, associates and other loved ones strengthens fortitude, enlivens patriotism, makes loneliness endurable and inspires to even greater devotion the men and women who are carrying on our fight far from home and from friends.”