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.

• Thales' Eclipse: Halted the epic Battle of Halys River, thereby
  saving one or both of the participants (Lydia and Media) from
  destruction.
• Kamikaze (Divine Wind): Created a storm that destroying the invading Mongol fleets, thereby saving Japan from foreign conquest.
• Athenian Typhoid: Wreaked havoc throughout Athens, contributing to its downfall in the Peloponnesian War.
• Bering Land Bridge: Facilitated the "invasion" of North America.
• Clouds over Kokura: Obscured the primary target for the "Fat Man"
  atomic bomb, thereby saving Kokura but resulting in the destruction of
  Nagasaki.
• Legend of Quetzacoatl: Convinced the Aztecs that Cortez was the
  reincarnation of Quetzacoatl, thereby facilitating the Spanish conquest
  of Latin America.

For more information, read:

Darwin's Origin of Species

Herodotus' Histories

Mitchell's Eclipses of the Sun

Lamont-Brown's Kamikaze

Daniels' Almanac of World History



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