system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the United States Merchant Marines?

How do they differ from private shipping companies, as they seem to fulfill the same purpose?
gpt
In peacetime, the Merchant Marine transports cargo and passengers no differently than any other shipping company.

In wartime, the Merchant Marine transports soldiers and material for the military. The Department of Defense can also commission Merchant Marine officers as military officers.