So if a U.S. Navy vessel flies the old pirate flag when returning to port (and some do), it's all in good fun. No one thinks the submarine USS Jimmy Carter, which was spotted flying the pirate banner back in 2017, is actually out there terrorizing sea lanes and stealing doubloons.

The US? No I think most people would absolutely assume they're out there terrorising people

*Only one US warship, Royal Navy submarines have been using it for over a century.

https://www.military.com/history/only-navy-warship-authorized-fly-pirate-flag-sea.html

The Only Navy Warship Authorized to Fly a Pirate Flag at Sea

Although there are dozens of active Arleigh Burke-class guided missile destroyers in the U.S. Navy's fleet, there's something special about the USS Kidd. It doesn't have any secret technology or weapons (that we know of), and its capabilities are the same as any other of its class. The standout feature that tends to capture the attention of even the most landlubberly civilian is the massive Jolly Roger that the Kidd is often seen flying from its mast.

https://www.military.com/history/only-us-navy-warship-authorized-fly-foreign-flag-sea.html

The Only US Navy Warship Authorized to Fly a Foreign Flag at Sea

You might be thinking to yourself, "Didn't we fight a whole war to keep American sailors from working under a British flag?" Well, it's been a long time since the War of 1812, and relations between the United States and the United Kingdom have changed significantly since then. Not only is this warship the only vessel in the U.S. Navy's fleet to fly a foreign flag, but it's also the only ship with an officer from Britain's Royal Navy aboard as a permanent member of the ship's company to ensure proper navigation -- all meant to honor the "British Bulldog," Winston Churchill.

Commissioned in 2001, the Arleigh Burke-class destroyer USS Winston S. Churchill was named for the famous prime minister who led Britain through World War II and has been permitted to fly the Royal Navy's White Ensign -- a red St. George's Cross on a white field with a Union flag in the canton -- during special occasions. 

https://www.navy.mil/USS-Constitution/Flags-and-Special-Events/

FLY A FLAG ABOARD USS CONSTITUTION!

USS CONSTITUTION's Flag Coordinator honors requests to fly National Ensigns (a.k.a. American Flags) of recognized current and historic configurations, U.S. Navy Jacks and other official flags of military branches, state and federal government agencies.

That moment when you spot a US Navy formation and one warship is sailing under a skull-and-bones pirate flag, one is sailing under a British naval flag, and the third is flying what appears to be the Oklahoma Peanut Commission logo.

It doesn't have any secret technology or weapons (that we know of)

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