A British incursion into Afghanistan ended in disaster in 1842 when an entire British army, while retreating back to India, was massacred. Only a single survivor made it back to British-held territory. It was assumed the Afghans let him live to tell the story of what had happened.
The background to the shocking military disaster had been the constant geopolitical jockeying in southern Asia which eventually came to be called “The Great Game.” The British Empire, in the early 19th century, ruled India (through the East India Company), and the Russian Empire, to the north, was suspected of having its own designs on India.
The British wanted to conquer Afghanistan to prevent the Russians from invading southward through the mountainous regions into British India.