I mean the island of Sark was unsuccessfully invaded by a single dude in 1990 (apologies for the terrible linked source, its just one of the first search results that wasn't a random reddit thread or other even less reputable source)

The local volunteer Constable, the only law enforcement on the island, being informed of the invasion thanks to the notices, decided to take the threat seriously. On the morning of the invasion, Constable Perrée began to plan the island’s defense, which was a two stage scheme. First, he went looking for Mr. Gardes. He found him sitting on a park bench in army fatigues preparing for the massacre that was about to befall this island. As the Constable approached the invading army (a.k.a. Gardes), he saw Gardes loading his automatic weapon.

The first “showing up” stage now complete, Mr. Perrée executed the second stage of the plan. He complimented Mr. Gardes on his choice of gun and convinced Gardes to remove the magazine and let him see the instrument of the island’s assured defeat, so the constable could better admire it. When Gardes acquiesced, the Constable took the gun and punched him in the nose. The invasion was over. The gun used in the attempted coup now sits in the Sark Museum next to old ships and a dedicated exhibit to one of the island’s two original telephone calls.

Edit: I'm a little disappointed there isn't a Wikipedia article on the subject with a list of belligerents, details of the strength of forces and a tally of the casualties and losses like this:

Well, there was Operation Red Dog by 13 (3 being agents so really more like 9-10) people who tried to take over Dominica and make it a white ethnostate...