Napoleon Invading Russia
In June 1812, Napoleon invaded Russia with one of the largest armies ever assembled for battle, and was so confident of his impending victory he wagered the war wouldn't last more than 20 days [source: PBS]. It wasn't Russia Napoleon wanted, necessarily (although Napoleon and Czar Alexander I were at odds over trade with England); it was India. But due to lice infestations and subsequent typhus infections, food shortages, freezing temperatures and, eventually, Russian troops, the Grande Armee wouldn't make it beyond Moscow.
More than 600,000 men from Napoleon's empire marched toward Russia, but just a few more than 100,000 were left fighting by early September 1812, and in the end Napoleon was escorted by Russian troops back to France [source: Knight].