In 1912, Sir Arthur Conan Doyle wrote a short story about how a small but warlike nation would conquer Britain. It would do so using a new technology in a new and brutal way. It shocked the nation and, three years later, it became a reality.
Arthur Conan Doyle was making a point when he wrote a story with the catchy name of Danger! (The exclamation point is included in the title and well-deserved by the story.) The story was part of a genre of fiction that focused on how England was unprepared for the inevitable war with Germany. Doyle's story focused on the nation's vulnerable Navy, and on the emergence of submarine warfare.
At the beginning of the story, the fictional nation of Norland declares war. The British public is not greatly alarmed, because Norland seems to be an inconsequential opponent. Norland's submarines effect a blockade, sinking all merchant ships that venture near England. Once the blockade of merchant ships is complete, they take the next step, sinking the Olympic, a massive luxury passenger vessel. The Norland technology is sufficiently advanced that the navy can do this with a single torpedo. England surrenders.
Doyle doesn't make the ending as grim as it could be. Norland is only a "temporary conqueror," because the country is "too weak to reap the fruits of her victory." England isn't engulfed in an empire, but it has to settle on unfavorable terms after a costly war. Doyle warns the reader, "Had we endured this humiliation at the hands of any of the First-Class powers it would certainly have entailed the loss of all our Crown Colonies and tropical possessions, besides the payment of a huge indemnity."
The story wasn't subtle, but it was right in its understanding of the changing face of naval warfare — an understanding shared by military leaders. Submarines were a threat not only because they could take out merchant vessels but because it was impossible for a submarine to stick around and save the crew of the vessel, as had been the custom up until that time. A submarine simply had no place to put the crew of a merchant ship, let alone the population of a passenger vessel. When a German submarine torpedoed the passenger vessel Lusitania in 1915, killing over a thousand people, Doyle's story seemed to be coming true. (Although as was found out years later, the Lusitania was carrying arms, and so was technically a legitimate target.)
In this war, Britain and its allies prevailed, but the ending wasn't Doyle's greatest error. What Doyle's story really got wrong was the scope of the devastation that even the victors of the war faced.