Battle of Waterloo
In earlier days, Napoleon seemed unstoppable until two separate campaigns failed.  He gathered a huge army to invade and to conquer Russia in 1812.  His army was caught by the Russian winter and destroyed.  The countries of Eastern Europe, led by Austria and Prussia, began to ally against him, forcing his troops back towards France.  Meanwhile, a small army in Portugal and Spain, led by Arthur Wellesley, who later became Duke of Wellington, began to push Napoleon’s troops out of Spain.