What makes you think this doesn't happen already. Arizona called in the National Guard on California during construction of tunnels out of Lake Mead. Mexico has threatened to take the US to international court over the salinity of the Colorado River. Dynamite was set off numerous times on the aqueducts that bring water to LA. Hostages have been taken and killed over the construction of dams in South America. Fresh water already is the scarcest resource we have in the American West. Far scarcer than oil by a long shot.