On September 14, 1812, the French Grande Armee marched into Moscow. The city had been abandoned by most of its population and by its government, and the fires began that night and the next day. Napoleon watched the burning from the Kremlin and waited five weeks for Tsar Alexander I to sue for peace. The tsar did not.
The decision to burn Moscow was not a single order. Kutuzov had made up his mind after Borodino to give up the city without another battle, on the calculation that the army mattered more than the capital, and he had said so at Fili on September 13. Fyodor Rostopchin, the governor general, had spent the previous weeks preparing the ground: the fire pumps were removed or disabled, the prisons were emptied, and the city's stores were either taken or destroyed. The detail that turned the retreat into an inferno was the removal of the pumps, because a wooden city with no way to fight a fire will burn until it rains.
The scale was enormous. Moscow was the largest city in Russia, with around 250,000 people before the war, and most of it was timber. Something like three quarters of the buildings were destroyed. The French lost the supplies they had expected to winter on, which was the whole point of taking the city, and the army had already been living off the country for two months on a line of communication that ran for a thousand kilometres through partisan country.
Napoleon's mistake was not entering Moscow. It was staying. He had taken the capital of every other country he had fought and had got a negotiation out of it, and he kept sending peace feelers through October while his men looted the ruins and the weather held. He left on October 19, too late, tried to retreat southwest through country that had already been stripped, and was forced back onto the road he had come by.
The retreat is what destroyed the Grande Armee. Of the roughly 600,000 men who crossed the Niemen in June, a small fraction came back. The Grand Army that had won at Borodino a month before was already down to perhaps 100,000 effectives by the time it reached Moscow, and winter, the Cossacks and the Beresina crossing finished the rest.
Moscow was rebuilt within a decade, and the fire of 1812 is the event Tolstoy built War and Peace around.