hi! not discouraging other responses, but fyi, there have been a few questions about natural phenomena occurring during/before battles, so you may find some interesting examples in there

• how about dragons? "In this year terrible portents appeared in Northumbria... these were exceptional flashes of lightning, and fiery dragons were seen flying up in the air..." Is this account from the Anglo-Saxon chronicle a simple fabrication, or a misunderstanding of a natural phenomenon? and a bit more in How do historians account for presentism, bias and ideology in primary sources?

• Has there ever been a natural disaster (earthquake, volcano, flood) that took place during a military engagement?

• Have there been any natural disasters that have changed the tide of war between nations?

• Has there been cases where an army uses a body of water to flood another in the West? If not, why not?

• Has an invading army ever used a natural disaster as prelude to their invasion?

• While we're all hoping for the best with Hurricane Sandy, what are some of the great natural disasters of history with which us lay-folk might not be familiar?

• What natural disaster significantly changed the course of history?

• Why have almost all cultures in ancient history uniformly perceived comets as a sign of death, destruction and plague?

• How where eclipses (both Solar and Lunar) viewed and explained through history?

• Comparing the Battle of Rorke's Drift with the battle of Battle of Isandlwana (which took place the same day), and I have a question.

e: added a couple more

The most famous example is the final battle of the war between the Lydians and Medes which helped secure peace between the two rivals. An eclipse occured in the middle of the battle, taken as a sign against the conflict. Because of this, assuming sources are correct, we can date the battle precisely to May 28, 585 BC.

This is, of course, far from the only instance, but probably the most well known. Another is the so called 'Miracle of the Rain' during the Roman Empire's Marcomannic Wars, where a dehydrated and demoralized Roman Army was suddenly reinvigorated by a rain storm that also supposedly wrought havoc on the German lines, including lightning strikes.