A decade later, a greater drama ensued when a group of Germans with plans to build a luxury casino on the platform tried to take control of Sealand while Mr. Bates and his wife were away. They held Michael Bates hostage for several days before Roy Bates stormed Sealand and retook it in a dramatic helicopter raid. He imprisoned one of the men there. When the German government sought Britain’s help in freeing him, Britain declined to intervene, citing the 1968 ruling.

Germany sent a diplomat, the man was eventually freed, and Mr. Bates asserted that Germany had effectively recognized Sealand as a sovereign nation.

Even after Britain expanded its territorial waters to 12 miles from shore, it mostly left Sealand and the Bateses alone. The family has explored various means of economic development, including housing an Internet company that wanted to create a financial haven without government oversight. It is still considering playing host to an online casino. WikiLeaks is said to have considered moving its servers there.

For now, most of Sealand’s trade is driven by Roy Bates’s grandson James — Prince Royal James — who oversees the Sealand Web site.

“The history of Sealand is a story of a struggle for liberty,” the Web site says. “Sealand was founded on the principle that any group of people dissatisfied with the oppressive laws and restrictions of existing nation-states may declare independence in any place not claimed to be under the jurisdiction of another sovereign entity.”

Paddy Roy Bates was born on Aug. 29, 1921, in London. His father served in the Royal Artillery in World War I and suffered lung damage from being gassed. The family moved to Essex with the goal of improving his health. According to an account on the Sealand Web site, Roy Bates was the only one of five siblings who survived childhood, and he barely survived his 20s, suffering several war wounds as a British soldier.