In anticipation of Napoleon's advance into Moscow, Russian troops had taken food, provisions, and a large segment of the population and then fled Moscow to St. Petersburg.
There remains some disagreement on whether the retreating Russian troops had also set fires to further destroy any remaining food or supplies. The fires eventually burned Moscow to the ground leaving Napoleon's already starving Grande Armée in even more desperate straits.
The other theory says that Napoleon's Grande Armée upon finding no surrendering Russian troops, and no expected victor's provisions from what Napoleon viewed at the time as a defeated Russian army; and in an attempt to stay warm, may have started wood-burning fires themselves which then unfortunately spread and burned Moscow to the ground.
Russian Tsar Alexander I signed a manifesto in December 1812 stating that he intended to build a Cathedral thanking God for saving Russia from Napoleon's Grande Armée and as a memorial to the sacrifices of the Russian people.
The Tsar commissioned the building of the beautiful and incredibly ornate Cathedral of Christ the Saviour in Moscow but it would undergo a change in the building site; some architectural plan changes, implemented by one or two new Tsars before the Cathedral of Christ the Saviour would host the debut of Tschaikovsky's 1812 Overture in 1882.
As the Cathedral of Christ the Saviour was finally nearing completion, a friend of Tschaikovsky's had suggested that he compose something which would commemorate the 1812 Russian victory over Napoleon's Grande Armée; commemorate and be performed in celebration of the completion of the Cathedral of Christ the Saviour; commemorate the 25th anniversary of the coronation of Tsar Alexander II in 1881 [Note: After many failed assassination attempts, Tsar Alexander II was assassinated March 13, 1881] as well as coinciding with the beginning of the 1882 Moscow Arts and Industry Exhibition.
Tchaikovsky began the project on October 12, 1880 and completed the 1812 Overture six weeks later.
Great post! Thanks!