In June 1812, Napoleon invaded Russia to compel Emperor Alexander I to remain in the Continental System. The Grande Armée, consisting of as many as 650,000 men (roughly half of whom were French, with the remainder coming from allies or subject areas), crossed the Neman River on 23 June 1812. Russia proclaimed a Patriotic War, while Napoleon proclaimed a "Second Polish War". But against the expectations of the Poles, who supplied almost 100,000 troops for the invasion force, and having in mind further negotiations with Russia, he avoided any concessions toward Poland. Russian forces fell back, destroying everything potentially of use to the invaders until giving battle at Borodino (7 September) where the two armies fought a devastating battle. Despite the fact that France won a tactical victory, the battle was inconclusive. Following the battle the Russians withdrew, thus opening the road to Moscow. By 14 September, the French had occupied Moscow but found the city practically empty. Alexander I (despite having almost lost the war by Western European standards) refused to capitulate, leaving the French in the abandoned city of Moscow with little food or shelter (large parts of Moscow had burned down) and winter approaching. In these circumstances, and with no clear path to victory, Napoleon was forced to withdraw from Moscow.
So began the disastrous Great Retreat, during which the retreating army came under increasing pressure due to lack of food, desertions, and increasingly harsh winter weather, all while under continual attack by the Russian army led by Commander-in-Chief Mikhail Kutuzov, and other militias. Total losses of the Grand Army were at least 370,000 casualties as a result of fighting, starvation and the freezing weather conditions, and 200,000 captured. By November, only 27,000 fit soldiers re-crossed the Berezina River. Napoleon now left his army to return to Paris and prepare a defence of Poland against the advancing Russians. The situation was not as dire as it might at first have seemed; the Russians had also lost around 400,000 men, and their army was similarly depleted. However, they had the advantage of shorter supply lines and were able to replenish their armies with greater speed than the French, especially because Napoleon's losses of cavalry and wagons were irreplaceable.