Sealand?
Emphasis where it appears is mine.
> In 1943, during the Second World War, HM Fort Roughs [Sealand] was constructed by the United Kingdom as one of the Maunsell Forts, primarily for defence against German mine-laying aircraft
> On 2 September 1967, the fort was occupied by Major Paddy Roy Bates, a British subject and pirate radio broadcaster, who ejected a competing group of pirate broadcasters.
> In 1968, British workmen entered what Bates claimed to be his territorial waters in order to service a navigational buoy near the platform. Michael Bates (son of Paddy Roy Bates) tried to scare the workmen off by firing warning shots from the former fort.
> In August 1978, while Bates and his wife were in England, Alexander Achenbach, who describes himself as the Prime Minister of Sealand, hired several German and Dutch mercenaries to spearhead an attack of Roughs Tower. They stormed the tower with speedboats, jet skis and helicopters, and took Bates' son hostage. Bates was able to retake the tower and capture Achenbach and the mercenaries.
And of course...
> While it has been described as the world's smallest country, the world's smallest nation, or a micronation, Sealand is not currently officially recognised by any established sovereign state, although Sealand's government claims it has been de facto recognised by the United Kingdom (after an English court ruled it did not have jurisdiction over Sealand as territorial water limitations were defined at the time) and Germany
Both of those claimed recognitions took place after an act of agression toward British / German citizens mentioned in the above excerpts.
So you've got a platform in the ocean that was created as part of a war effort, that has been the subject of a failed and a successful military coup, whose only unofficial recognition by established states came after assaulting their citizens, and that is still not officially recognized by any other state. (And which is currently sitting in British territorial waters, after they unilaterally annexed its surrounding ocean.) And all this is for a "state" with a population estimated at "over 50".