Whether they are seen as acts of God, or as simple climate-related
occurrences, natural events have always had a sizeable impact on
military operations. At the small end of the scale are the little changes in terrain or weather that may affect a battle or a small war. For
example, many armies have postponed their campaigns due to inclement
weather conditions, and many militaries have suffered from rampant
disease. On the other end of the scale are the
times when nature has so much of an impact that the fate of an entire
nation or civilization is decided upon it. In
the words of Charles Darwin, these are times when “the war of nature”
results in the downfall of one party and the rise of another.

• Colonization Smallpox: Rampant disease severely weakened the
  Aztecs and Incas, allowing small bands of Spanish conquistadors (led by
  Cortez and Pizarro, respectively) to easily overthrow two great empires.
• Revolutionary Wind and Fog: Heavy winds subsided after the Battle
  of Long Island, allowing American troops to evacuate and fight another
  day.  Their retreat was concealed by a dense fog.  Later, just before
  the Battle of Trenton, a heavy fog concealed the Americans long enough
  to conduct a surprise attack which greatly boosted the morale of the
  Continental Army.
• Russian Winter: Cold temperatures forced Napoleon to retreat
  after he failed to conquer Russia and find accommodations in Moscow. 
  The lack of grass and unfrozen roads resulted in the destruction of up
  to 75% of Napoleon's Army as it marched back to France.

For more information, read:

Hopkins' The Great Killer

Diamond's Guns, Germs, and Steel

McCullough's 1776

Burton's Napoleon's Invasion of Russia

Tolstoy's War and Peace

George's Napoleon's Invasion of Russia



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