It's not as though conflicts over water are an entirely new phenomenon. The Pacific Institute keeps a running list of water conflicts [PDF] that stretches back 5,000 years. The first human-on-human conflict over water occurred around 2500 BC in Mesopotamia, according to the list.
A Mesopotamian city state, Lagash, diverted water from its neighbor, Umma. The most recent water conflict: In 2008, the Taliban threatened to blow up Pakistan's Warsak Dam. (The list hasn't been updated for a year.)
Some see evidence of increased risk of conflict in a warming world where some regions are drying.
A report titled “Rising Temperatures, Rising Tensions: Climate change and the risk of violent conflict in the Middle East,” which was released earlier this year by the International Institute for Sustainable Development, found that after the 2007-'08 drought in Syria, residents abandoned 160 villages.
Rainfall in the area has diminished markedly in the past 50 years, probably due to global warming. In Syria alone, some 300,000 farmers and herders abandoned their homes, families in tow, for urban camps because of the drought. Around 800,000 lost their livelihoods entirely