Napoleon once said "I love power, as a musician loves his violin". His love for power ultimately led him to his death. "All power tends to corrupt and absolute power corrupts absolutely" - Lord Acton.
The Continental System, In November 1806. This was Napoleons first mistake. He signed a degree ordering a blockaded which prevented communication and trade from Great Britain and other European nations and he called the Continental System. Unfortunately this blockade was not nearly as tight enough. Smugglers brought in cargo, and at times napoleon's allies disregarded his orders. This led to Britain developing a blockade and the U.S. declared war on 1812.
The Peninsular War, in 1808. Because Portugal was ignoring the continental system, napoleon sent an army through Spain to invade them in 1808. The move outraged Spanish people and they remained loyal to there former monarchy, helping Portugal with the battle causing napoleon to lose bout 300,000 men.
The Invasion of Russia, in 1812 was Napoleons greatest mistake. Napoleon decided to invade Russia because napoleon ex ally the Russian czar refused to stop selling grain to Britain. June 1812 napoleon and his army marched into Russia, Russia retreated and pulled back which gave Napoleon Moscow. Before leaving Russians slaughters there live stalk so napoleon had nothing to eat, napoleon stayed in Moscow for five weeks expecting Czar to make a peace treaty but he never did. He decided to order his army to go back, though many had died from wounds, hunger, exhaustion, and cold napoleon had only 10,000 of his army left to fight.