Water has been used as a weapon, in both cold and warm regions, in both cold and warm weather. Generally, this is in the form of deliberate flooding, by breaking dikes, dams, or diverting rivers. The goal can be very local and specific, such as to flood a besieged city, or to flood an enemy camp (often the besiegers during a siege), or broader, to flood a large area to deny it to the enemy, to destroy crops, to destroy industry, to force civilians too move, to reduce the enemy's ability to supply their forces.

Some modern examples:

• In 1938, the Chinese Nationalists breached the dykes of the Yellow River to halt a Japanese offensive. While this did indeed slow down the Japanese, it also killed perhaps 500,000 civilians and displaced millions.

• In 1943, the British breached dams (Operation Chastise, the Dam Busters raids) causing flooding in the Ruhr valley, damaging German industry (through loss of hydroelectric power from the dams, regulation of water levels in canals, loss of clean water for industrial purposes, and direct damage to mines and factories due to flooding).

Earlier:

• Siege of Leiden, 1573-1574: After 10 months of siege, the Dutch broke dikes to flood the area, reducing the mobility of the Spanish forces, and their ability to supply their forces, and to allow the Dutch relief force to use boats/ships.

• In 1234, the Song Chinese celebrated the recent defeat of the Jin (earlier in the same year, by the Mongols and Song in alliance) by attacking the Mongols, with the main attack aimed at Kaifeng (which had been the Jin capital). Mongol forces in the area were weak (at least partly due to the lack of supplies in the area; Song food supplies had been essential for supporting the armies that completed the defeat of the Jin), and the Mongols abandoned Kaifeng and much of the surrounding area, and opened dikes to flood the area. The Song armies reached Kaifeng (and took it, as it was undefended), but had to march through waist or even neck deep water to reach it. No further supplies could be sent to support the army, and the Song had to retreat (or starve), and the Mongols recovered the area when the flood water receded.

Apart from the loss of mobility to an army and its supplies, flooding can be tremendously destructive to armies. Food supplies can be destroyed by soaking, the water can make it impossible to cook or to boil water, and (together with that last point), flooding eliminates sources of clean drinking water. This is especially bad if a besieged city is flooded, as the flood water merges with sewerage, garbage, dead bodies, etc. to create a stinking and putrid lake as the only source of drinking water. Unsurprisingly, the resulting disease casualties can destroy an army.

There is a long tradition of hydraulic warfare in China, and Ralph D. Sawyer, Fire and Water, Westview, 2004, with copious (and rather literal rather than easy-reading) translations from ancient sources, reviews this tradition.

See also, for a post-WW2 discussion of the possibilities: DZIUBAN, S. (1950). Implications of Artificial Flooding in Military Operations. The Military Engineer, 42(285), 13-15. http://www.jstor.org/stable/44561030