The Continental System
Napoleon's first mistake was in Nov. 1806, he ordered a blockade to prevent trade with Britain along with other European nations. This was a policy called the Continental System. Even with the blockade, the British still were able to get in. Since the British had a better naval force, they were able to make a better blockade work. In the end, Napoleon was hurt more when the economy was weakened. This system failed because the blockade wasn't sting enough, and the British navy was stronger than Napoleon's navy.
The Peninsula Campaign
The second mistake was the Peninsula Campaign when Napoleon invaded Portugal through Spain. In 1808, Portugal was ignoring the Continental system and Napoleon went to invade. The Spanish townspeople rioted when Napoleon deposed the original king and put his brother Joseph in to rule. The Catholics feared that their church, which was under attack, would be undermined. The Spanish peasant fighters, guerrillas, they fought the French, and the British aided them. The Peninsula War caused Napoleon to lose 300,000 men. This attack failed after a few years because the nationalism of the people caused uprisings to help gain support against Napoleon.
The Invasion of Russia
The third mistake was the worst mistake Napoleon had made that led to his downfall. Alexander I and Napoleon were allies, but Russia refused to stop trade with Britain. Napoleon decided to invade Russia, and his troops weren't loyal to him because they were mistreated. When Russia retreated to Moscow, they had a scorched-earth policy that gave no food to the enemy. In the Battle of Borodino, the Russians retreated to Moscow, but Alexander I burned the city rather than surrender. When Napoleon was returning home with his starving soldiers, the Russian raiders attacked the group. While the temperature was 30 below zero, many died from hunger, exhaustion, wounds, and cold. Once they finally returned, there was only 10,000 soldiers left.