Napoleon combined the policymaking functions of head of state with the military strategy-making of armed forces commander, and there were both advantages and disadvantages of this combination. The primary advantage was that Napoleon did not have to collaborate with anyone else to make decisions in either office; he could make his own policies and develop his own military strategies without answering to anyone else. His approach also created a direct tie between the state and the military, so that there was no bureaucratic red tape separating them and slowing him down in either office. On the other hand, there were also disadvantages to Napoleon's approach. For one thing, with no one to dispute his decisions as head of state and military commander, this meant that he was free to make horrendous errors. He did not have to listen to advisors, because he was the only decision-maker. In addition, since there was no separation of the branches of the government, there were no governmental checks and balances; a tyrant could take over and create vast destruction.
Historian Robert Burnham asserts that Napoleon failed because of "faulty logistics, poor discipline, disease, and not the least, the weather." Burnham notes that Napoleon's warfare tactic was to concentrate his forces in a key place, moving them there as quickly as possible, but this did not work well in Russia, where the network of roads was abysmal. The poor discipline was due to troops having to forage off the land to survive, taking them far from the camp (Burnham). Disease was rampant because of poor diet and fatigue (Burnham). The weather ranged from too hot to too cold, and when it rained, the roads became "quagmires of mud" that were hard for Napoleon's troops to get through" (Burnham).
Burnham, Robert. "Why Did Napoleon Fail in Russia in 1812?" The Napoleon Series. FAQ.