A fire that started in a barn on the south-west side of Chicago on the evening of October 8, 1871 burned for two days and destroyed the middle of the city. About 300 people died. Some 17,000 buildings went, and around 100,000 residents, a third of the population, were left without homes. The burned area covered roughly three and a half square miles and took in the business district, the courthouse and the waterworks. What carried it that far was wind: a strong south-west gale threw burning debris over the river, and then over the south branch, faster than anyone could clear a break.
The city was tinder. Chicago had grown from 4,000 people to 300,000 in thirty years, and most of it was wood, with wooden houses, wooden sidewalks, wooden pavements, tarred roofs and lumber yards stacked along the branches of the river that ran through the middle of town, and less than an inch of rain had fallen between July and October. The fire department had spent the previous night on a large fire and its men and horses were exhausted when the alarm came in.
The famous explanation is wrong. The story that Catherine O'Leary's cow kicked over a lantern was printed by a Chicago newspaper and repeated everywhere; a reporter on that paper said years afterwards that he had made it up because it made a good story. The official investigation could not settle on a cause and said so. The barn belonged to the O'Learys and the fire did start there, but nobody established how.
Two other fires burned the same day: the Peshtigo fire in Wisconsin, which killed at least 1,200 people and is still the deadliest fire in American history, and the Great Michigan fire, which burned Holland and Manistee. Rain on the third day put the Chicago fire out. Rebuilding began before the rubble was cleared, and the new downtown went up in stone and brick, which is why the Chicago school of architecture and the first steel-framed skyscrapers appeared there in the 1880s rather than anywhere else.