Today, as in Iraq and to a lesser extent Afghanistan, wars typically invite accusations that the war-fighting nations are interested in oil and gas deposits. Whether this is true, at least in those conflicts, is debatable. In the coming century or so, however, it is certain that there will be at least serious international stress, if not outright armed conflict, over what may become an even more precious scarce resource: fresh water.
The Earth does not suffer from a shortage of water, but most of it comes in the form of saltwater, and can be used only for drinking and irrigation purposes following expensive and time-consuming desalination procedures. Moreover, as the human population increases and poor regions develop, rich sources of fresh water, such as aquifers and large rivers, are being increasingly intensively exploited at the local, regional, and national levels. Particularly in arid and water-poor regions such as the Middle East and parts of Africa, therefore, it becomes quite conceivable that at some future date nations with insufficient water reserves will feel driven into conflict over shared, scarce water sources such as strategically located lakes and rivers.
In relatively large and industrialized Western nations, and particularly in the United States and Canada, water wars seem almost inconceivable. However, even within these nations, it is easy to see the roots of potential conflict - albeit, in these cases, either internal to a nation (and therefore dealt with at the level of state and national governmnet) or resolved through international diplomacy. Important Canadian-American boundary waters, like the Columbia River in the northwest and the Great Lakes in the east, are covered by international treaties. The great Colorado River staggers to the sea in the form of a trickle, but while those downstream in the U.S. do suffer from the actions of those upstream, the resulting stresses can at least theoretically be resolved through internal political compromise rather than open conflict.
This is not necessarily the case where such resources cross multiple international borders, in areas already prone to violent conflict. For example, Israel, Jordan, Lebanon, and - should it ever become an independent state - Palestine must juggle separate national uses of the Jordan river, while Syria, Turkey and Iraq must divide between themselves the waters of the Euphrates and the Tigris. Some of these countries have already fought violent conflicts in the recent past; at the very least, if population levels continue to rise and available water continues to fall, water shortages will add further tension to an already critical situation. Optimistically, negotiation over water might pave the way for broader peace talks; more pessimistically, those countries with temporary strategic advantages in a given situation might decide to take advantage of the opportunity to ensure continued and privileged access to a shared water supply.
In some cases, over-use has already led large water sources, like the Aral Sea in Asia, to shrink measurably - causing increased fears that what water remains might be fought over. Lake Chad, a massive African lake which is shared by Chad, Niger, Nigeria, and Cameroon, has also suffered serious depletion and there may be future conflicts, including violent ones, over how to use the remaining water.