The French invasion of Russia, also known as the Russian campaign, the Second Polish War, the Second Polish campaign, the Patriotic War of 1812, and the War of 1812, was begun by Napoleon to force Russia back into the Continental blockade of the United Kingdom. Napoleon's invasion of Russia is one of the best studied military campaigns in history and is listed among the most lethal military operations in world history. It is characterized by the massive toll on human life.On 24 June 1812 and the following days, the first wave of the multinational Grande Armée crossed the Niemen into Russia. Through a series of long forced marches, Napoleon pushed his army of almost half a million people rapidly through Western Russia, now Belarus, in an attempt to destroy the separated Russian armies of Barclay de Tolly and Pyotr Bagration who amounted to around 180,000–220,000 at this time. Within six weeks, Napoleon lost half of the men because of the extreme weather conditions, disease and hunger, winning just the Battle of Smolensk. The Russian Army continued to retreat, under its new Commander in Chief Mikhail Kutuzov, employing attrition warfare against Napoleon forcing the invaders to rely on a supply system that was incapable of feeding their large army in the field.

The fierce Battle of Borodino, seventy miles (110 km) west of Moscow, was a narrow French victory that resulted in a Russian general withdrawal to the south-east of Moscow. On 14 September, Napoleon and his army of about 100,000 men occupied Moscow, only to find it abandoned, and the city was soon ablaze, instigated by its military governor. Napoleon stayed in Moscow for five weeks, waiting for a peace offer that never came. Because of the nice weather he left late and hoped to reach Smolensk by a detour. Losing the Battle of Maloyaroslavets he was forced to take the same route as he came. Lack of food and winter clothes for the men, fodder for the horses, and guerilla warfare from Russian peasants and Cossacks led to greater losses. Again more than half of the men died on the roadside of exhaustion, typhus and the harsh continental climate. Heavy loot was thrown away; artillery was left behind.

In early November it began to snow, which complicated the retreat. In the Battle of Krasnoi Napoleon was able to avoid a complete defeat. Meanwhile, he was almost without cavalry and artillery, and deployed the Old Guard for the first time. When the Berezina was reached, Napoleon only had about 49,000 troops and 40,000 stragglers of little military value. The Grande Armée had deteriorated into a disorganized mob, and the Russians could not conclude otherwise. On 5 December, Napoleon left the army in a sledge and returned to Paris. Within a few days, 20,000 more perished from the bitter cold and lice. Murat and Ney, the new commanders continued, leaving more than 20,000 men behind in the hospitals of Vilnius. What was left of the main armies crossed the frozen Niemen and the Bug disillusioned.

Although estimates vary because precise records were not kept, numbers exaggerated and auxiliary troops not always counted, Napoleon's army entered Russia with more than 450,000 men, more than 150,000 horses, around 25,000 wagons and more than 1,250 pieces of artillery. Only 120,000 men survived (excluding early deserters); as many as 380,000 died in the campaign. Perhaps most importantly, Napoleon's reputation of invincibility was shattered.