Monmouth was the lead ship of a class of ten armoured cruisers, each named for a British county, known together as the County Cruisers. Hers was Monmouthshire, the Welsh county, and she was laid down on the Clyde at Govan in 1899. She was a substantial ship: 463 feet long, designed to displace 9,800 tons, driven by triple-expansion engines fed by 31 boilers, and armed with fourteen six-inch guns. But she was never quite the ship she was meant to be. Monmouth was one of three in her class that failed to reach their designed speed, and several of her guns sat so low on the main deck that they could only be fired in calm weather. She had spent years on the China Station, then come home in 1913 to the reserve fleet, an aging cruiser easing toward the end of her service.

The war recalled her. Mobilised on 4 August 1914, Monmouth was recommissioned and crewed with around 735 men, a company of ratings, officers, 25 musicians of the Royal Marine band, and 69 Royal Marines. They were sent first to hunt German raiders off the Cape Verde and Canary Islands, then detached to patrol the Brazilian coast, and finally drawn south into the search for Vice Admiral Maximilian von Spee's German East Asia Squadron. By October she had joined the small squadron of Rear Admiral Christopher Cradock, ranging up the Chilean coast as far as Valparaiso. They were chasing some of the finest warships in the German navy with a collection of older, slower vessels, and the men aboard them surely sensed the mismatch.

The two squadrons found each other on the evening of 1 November, off Coronel. Cradock tried to use the setting sun to blind the German gunners, but Spee, faster and more powerful, refused to close until the light turned against the British. As the sun set at 18:50 it silhouetted Cradock's ships against the bright sky while the German vessels melted into the dark coastline behind them. At 19:04 the Germans opened fire. Their gunnery was deadly and accurate. Within minutes Scharnhorst was tearing into the flagship Good Hope while Gneisenau hammered Monmouth, and a shell soon blew the roof off Monmouth's forward turret, touching off an ammunition explosion that ripped the turret clean off the ship. She was burning and beginning to slow, and the night had only started.

What happened next was a slow, brutal end. The light cruiser Glasgow found Monmouth listing and down by the bow, fires out, struggling to turn her stern into the heavy swell, and could do nothing for her without dying alongside her. Glasgow slipped away into the dark. Later the German light cruiser Nürnberg came upon the crippled ship and closed to within 600 yards, shining a spotlight on her flag, waiting for her to surrender. Monmouth would not strike her colours. With her list now so steep that her port guns could not bear, she turned toward Nürnberg as if to fight, or perhaps to ram. Nürnberg opened fire again, and at 21:58 Monmouth rolled over and sank, taking every man aboard into the sea. Across both lost British cruisers, more than 1,600 men died that night, Admiral Cradock among them. A month later the Royal Navy avenged them at the Falkland Islands, destroying most of Spee's squadron. But nothing brought back the men of the Monmouth, who lie still where she went down, off the coast of Chile.