The Coast Guard is one of the 5 branches of the Armed Forces of the United States. (Army, Navy, Air Force, Marines, and Coast Guard)

The Coast Guard, however, is unique among the others in that it operates as part of the Department of Homeland Security in times of peace, and the Navy during times of war. In that respect, they are the only branch of the armed forces that has Law Enforcement responsibilities.

“But wait!”, you are thinking as you reflect on your high-quality public school education, “Doesn’t the Posse Comitatus Act expressly prohibit the armed forces of the US from engaging in Law Enforcement activities inside the US Borders?” You are correct. However, the Coast Guard is exempt from this act.

The Coast Guard traces its roots back to 1790, when Treasury Secretary Alexander Hamilton created the Revenue Marine as a department under the US Treasury.

You see, the early Federal Government relied upon tariffs for much of its funding. No one likes to pay the tax man, so people started smuggling. In turn, the Revenue Marine was created to crack down on smugglers and make sure that the new Federal Government got its share.

The Revenue Marine actually predates the US Navy. We did have a Navy during the Revolutionary War (The Continental Navy), but it was disbanded in 1790. So from 1790 until 1798, the Revenue Marine was the only official US presence at sea.

Some more Coast Guard key events:

• 1848 – Congress creates The Life-Saving Service and gives them a $10,000 budget. The Life-Saving Service operates on the East Coast and is run mostly by volunteers. Their mission is to… save lives.
• 1862 – The Revenue Marine is renamed the Revenue Cutter Service. (A cutter is a type of ship.) Plus “cutter” sounds mean. And as the Dread Pirate Roberts taught us, a good name is key.
• 1910 – Congress creates the US Lighthouse Service to oversee all of the Lighthouses in the US.
• 1915 – The Life Saving Service and Revenue Cutter Service merge and are renamed the US Coast Guard.
• 1939 – The Lighthouse Service becomes part of the Coast Guard.
• 1946 – The Coast Guard absorbs the Bureau of Marine Inspection and Navigation, which is responsible for inspecting ships for safety.
• 1967 – The Coast Guard becomes part of the Department of Transportation instead of the Treasury Department.
• 2003 – Coast Guard becomes part of DHS.

In case you’re wondering (and I know you are) about the relationship between the Navy and The Coast Guard, it’s about what you’d expect from two branches of the Armed Services. The Navy considers the Coast Guard a “Hooligan’s Navy” because of their reliance on enlisted personnel. The designation goes back to WWII when the Coast Guard basically took everyone who’d ever even looked at a ship and tried to turn them into sailors.

Source: Some wiki, but mostly this PowerPoint from the US Coast Guard Historian