The fire killed approximately 300 people, destroyed roughly 3.3 square miles (9 km2) of the city including over 17,000 structures, and left more than 100,000 residents homeless.

A long period of hot, dry, windy conditions, and the wooden construction prevalent in the city, led to the conflagration spreading quickly.

The fire is said to have started at about 8:30 p.m. on October 8, in or around a small barn belonging to the O'Leary family that bordered the alley behind 137 W. DeKoven Street.

[1] The fire's spread was aided by the city's use of wood as the predominant building material in a style called balloon frame.

More than two-thirds of the structures in Chicago at the time of the fire were made entirely of wood, with most of the houses and buildings being topped with highly combustible tar or shingle roofs.

[6] Compounding this problem, Chicago received only 1 inch (25 mm) of rain from July 4 to October 9, causing severe drought conditions before the fire, while strong southwest winds helped to carry flying embers toward the heart of the city.

When firefighters finally arrived at DeKoven Street, the fire had grown and spread to neighboring buildings and was progressing toward the central business district.

Firefighters had hoped that the South Branch of the Chicago River and an area that had previously thoroughly burned would act as a natural firebreak.

Former Lieutenant-Governor William Bross, and part owner of the Tribune, later recollected his response to the arrival of Gen. Sheridan and his soldiers: "Never did deeper emotions of joy overcome me.

Many people who were left homeless after the incident were never able to get their normal lives back since all their personal papers and belongings burned in the conflagration.

In April 1872, the City Council passed the ordinance to establish the free Chicago Public Library, starting with the donation from the United Kingdom of more than 8,000 volumes.

Based on a religious point of view, some said that Americans should return to a more old-fashioned way of life, and that the fire was caused by people ignoring traditional morality.

[20] Almost immediately, the city began to rewrite its fire standards, spurred by the efforts of leading insurance executives, and fire-prevention reformers such as Arthur C. Ducat.

Its developer, Potter Palmer, secured a loan and rebuilt the hotel to higher standards, across the street from the original, proclaiming it to be "The World's First Fireproof Building".

A bronze sculpture of stylized flames, entitled Pillar of Fire by Egon Weiner, was erected on the point of origin in 1961.

Additionally, though the inhabitable portions of the building were destroyed, the bell tower of St. James Cathedral survived the fire and was incorporated into the rebuilt church.

In 1893 the reporter Michael Ahern retracted the "cow-and-lantern" story, admitting it was fabricated, but even his confession was unable to put the legend to rest.

[37] Despite this, the Chicago city council was convinced of Bales's argument and stated that the actions of Sullivan on that day should be scrutinized after the O'Leary family was exonerated in 1997.

[35][38] Anthony DeBartolo reported evidence in two articles of the Chicago Tribune (October 8, 1997, and March 3, 1998, reprinted in Hyde Park Media) suggesting that Louis M. Cohn may have started the fire during a craps game.

When Mrs. O'Leary came out to the barn to chase the gamblers away at around 9:00, they knocked over a lantern in their flight, although Cohn states that he paused long enough to scoop up the money.

At a 2004 conference of the Aerospace Corporation and the American Institute of Aeronautics and Astronautics, engineer and physicist Robert Wood suggested that the fire began when a fragment of Biela's Comet impacted the Midwest.

Wood argued that four large fires took place, all on the same day, all on the shores of Lake Michigan (see related events), suggesting a common root cause.

Eyewitnesses reported sighting spontaneous ignitions, lack of smoke, "balls of fire" falling from the sky, and blue flames.

[43] Meteorites are not known to start or spread fires and are cool to the touch after reaching the ground, so this theory has not found favor in the scientific community.

Moreover, if a fragment of an icy comet were to strike the Earth, the most likely outcome, due to the low tensile strength of such bodies, would be for it to disintegrate in the upper atmosphere, leading to a meteor air burst like the Tunguska event.

A common cause for the fires in the Midwest in late 1871 is that the area had had a dry summer, so that winds from the front that moved in that evening were capable of generating rapidly expanding blazes from available ignition sources, which were plentiful in the region.

The Peshtigo Fire remains the deadliest in American history[50] but the remoteness of the region meant it was little noticed at the time, because one of the first things that burned were the telegraph lines to Green Bay.

On October 9, 1871, a fire swept through the city of Urbana, Illinois, 140 miles (230 km) south of Chicago, destroying portions of its downtown area.