The micronation of the Conch Republic seceded from the United States in 1982. They declared war on April 23rd 1982 and surrendered 1 minute after the declaration so they could apply for foreign aid.
In 1968 a man built an island off the coast of Italy and declared himself the President. He ran a restaurant, a bar, a nightclub, and a post office on the island. The Italian government didn’t like this, seized control of the island, and literally blew it up the next year.
There is a micro nation in Nevada named Molossia that consists only of a house, front yard and back yard. It also declared war on east Germany.
A floating British base was abandoned until a rogue DJ moved there. He declared the floating fortress the Principality of Sealand. He raised a flag, named himself Prince and his wife Princess. Sealand withstood court challenges, and remains an independent nation to this day.