Russian renunciation of the Continental System (Napoleon's scheme of economic warfare against Britain during the Napoleonic Wars had been ruinous to Russia's economy), compounded by differences over French influence in Poland, Sweden, and the Balkans, caused Napoleon to invade Russia with his Grand Army of nearly 500,000 soldiers in June 1812. Outnumbered, the Russians retreated, destroying crops in their wake and stretching French supply lines thinner and thinner. After winning bitter battles at Smolensk (August 17) and Borodino (September 7), Napoleon's troops entered Moscow (September 14), only to find themselves holding a city burned and gutted by its fleeing inhabitants. The Russians under Mikhail I. Kutuzov (1745-1813) repulsed Napoleon's further advances and the czar refused to negotiate a truce. Nothing remained but retreat for the French, who were now seriously suffering from lack of food, severe cold, disease, and fatigue. The retreating French were decimated even more by Kutuzov's perusing forces and incessant Cossack attacks, especially while crossing the Berezina River on November 26-28, 1812. Napoleon left his war-weary, tattered troops in December to return to Paris to put down a rumored plot against him and raise a new army. Only about 30,000 French soldiers survived the invasion and the retreat.