From We Are The Mighty:

In October 1921, gangsters hit a mail truck in New York City, making off with .4 million in cash, securities, and jewelry – million dollars when adjusted for inflation. That wasn’t the only high-stakes robbery. Between April 1920 and April 1921 alone, thieves stole more than six million dollars in U.S. mail robberies – million when adjusted for inflation. So when the Postmaster asked the President for the Marines, the Commander-In-Chief was happy to oblige.