After signing the 1979 peace treaty with Israel, Egyptian President Anwar Sadat said his nation will never go to war again, except to protect its water resources.

King Hussein of Jordan identified water as the only reason that might lead him to war with the Jewish state.
Former United Nations Secretary General Boutros Boutros-Ghali warned bluntly that the next war in the area will be over water. [...]

The hidden factor
International law is inadequate in defining and regulating the use of shared water resources. Few agreements have been reached about how water should be shared. [...]
Since the Madrid conference in 1991, Palestine-Israel negotiations and the now frozen negotiations with Syria have always stumbled over the issue of sharing water.
With the Israeli army in control prohibiting Palestinians from pumping water, and settlers using much more advanced pumping equipment, Palestinians complain of “daily theft” of as much as 80% of their underground water. [...]
Ariel Sharon went on record saying that the Six Day War started because Syrian engineers were working on diverting part of the water flow away from Israel.
“People generally regard 5 June 1967 as the day the Six-day war began,” he said.
“That is the official date. But, in reality, it started two-and-a-half years earlier, on the day Israel decided to act against the diversion of the Jordan.

Adapted from “Middle East Water Wars”, a BBC News article by Adel Darwish, May 30th 2003