Before the age of email, instant messaging, texting, and even mass phone calls, communication from families to soldiers was much more difficult, as this 1917 article details:
“The time when the soldiers from the firing line did not get the home mail they were hoping for came at the end of one of the eighteen-day periods in which it was impossible to send any mail from America because there were no ships going over. There have been two such periods since our troops arrived in France.”
That was during World War I. My maternal grandfather delivered mail to the troops during the Korean War several decades later, and even then there were complications delivering the mail. Today, as you can imagine, the situation is significantly easier.
Interestingly, another excerpt from the article reveals the discrepancy between inbound and outbound letters: 450,000 letters per week to the troops, but only 376,000 letters per month from them — almost five times as many letters to the troops as from them.
Speeding Up the Mails for American Soldiers: Every Week 450,000 Letters Go to France, and Lack of Ships Has Complicated the Postal Problem — Cantonment Service Systematized
From Sunday, November 18, 1917