Final answer:
Napoleon's invasion of Russia failed due to a combination of the inexperienced composition of his army, the Russian scorched earth tactic, and the onset of a brutal winter that led to disease and a lack of resources, compelling a retreat that decimated his troops.
Step-by-step explanation:
The main reason for the failure of Napoleon’s invasion of Russia was a combination of logistical challenges and Russian strategic tactics. Napoleon's Grand Army, although large, was comprised mostly of non-French recruits from territories such as Italy and Germany, many of whom were inexperienced. Moreover, the Russian use of the scorched earth policy devastated Napoleon's supply lines. As the brutal winter set in and resources dwindled, diseases like dysentery and typhus ran rampant. Eventually, Napoleon was forced to retreat, with only a fragment of his army surviving the harrowing journey back to France. This disastrous campaign significantly weakened Napoleon's power and set the stage for the resurgence of anti-French coalitions and his eventual downfall.