At sea, the line between war and peace has always been thinner than on land, a fact nowhere better illustrated than the waters off Iceland in the middle years of the 20th century. British fishermen had been taking advantage of the rich fisheries off Iceland since the late Middle Ages, and had found themselves in conflict with the rulers of Iceland (Denmark up until WWII) over where they could fish for almost as long. Throughout the 19th and early 20th centuries, Denmark repeatedly tried to expand their control over the waters around Iceland past the 3 mile limit, while Britain, driven by the importance of their fishing fleet, successfully pushed back.
Things began to change in the postwar world. Iceland, newly independent, was more willing to go to the mat over the issue, while Britain was no longer the world's leading superpower. The first clash came in 1952, when Iceland attempted to expand its control over offshore fishing from 3 miles to 4. The British responded by prohibiting the Icelandic fishing fleet from landing its catch in Britain, its largest export market. The Soviets, seeking power in the growing conflict with the West, stepped in to buy the Icelandic fish, a move the US countered by entering the market for Icelandic fish and bringing Italy and Spain in as well. This essentially nullified the British embargo, and by 1956, Iceland had won a clean extension of its control.
But the Icelanders were not satisfied with the 4-mile limit, and in 1958, they passed a law to extend it to 12 miles despite opposition from the entirety of Western Europe. This time, the British decided to respond not with boycotts but by sending the RN to do the most basic of naval functions, ensuring that one's ships can operate even where someone else would rather they didn't. But they would not do this in the traditional way, because while it would have taken the RN about 10 minutes to annihilate the 5 gunboats of the Icelandic Coast Guard, doing that to a fellow NATO member would have caused some diplomatic problems. Instead, this operation, dubbed the "Cod War" after one of the major fish in question, would involve the Icelanders trying to arrest British trawlers, while the RN did its best to stop them without actually breaking anything.