By the opening years of the 20th Century, the battleship had become an increasingly baroque creature. In the fifty years since steel and steam replaced wood and canvas, designers of battleships had crammed them as chock full of guns of all different calibers as they could, mounted in a bewildering array of turrets, casemates, barbettes, and holes cut in the hull. Needless to say, this made something of a hash out of shooting, since every gun crew was more or less on their own; there was no point in even trying to coordinate the ranging efforts of two guns of two different calibers mounted at two different elevations on the ship. Meanwhile, the entire creaky edifice was powered through the ocean blue by an engine only marginally more sophisticated than the one used to push old Number 9 down the tracks.
On this date in 1906, every battleship then afloat became instantly obsolete when the British Royal Navy launched HMS Dreadnought. Powered by high-tech steam turbines, the Dreadnought had only one caliber of main armament. Her twelve-inch main guns, mounted in pairs in powered turrets, were directed by mast-mounted spotting stations that could observe the splashes caused by the fall of shot and transmit corrections to a central control station over the first electric fire direction system used in a capital ship.
Of course, every nation in the world immediately wanted one. Pretty soon, you were nobody if you didn't have a "dreadnought". Banana republics mortgaged the national wealth to have British shipyards make one just for them. They became too valuable to risk, such was the amount of money and national prestige tied up in them. Whole battlefleets would spend whole wars mouldering away in harbor, lest they actually be exposed to the risk of being sunk. In the forty years between their invention and their ultimate eclipse by the aircraft carrier and submarine, dreadnoughts squared off in high seas combat only a handful of times, and usually with fairly inconclusive results.
But they sure do look cool.