I agree normal trade benefits both (would not occur other wise) but many wars have been fought over control of limited resources. You know your history better than me, but I think the first such wars were fought of control over salt deposits more than 2000 years ago. Fresh water will soon be making wars. Many, me included, think US has made several wars over oil. Japan attacked Pearl Harbor in a despirate effort to end the US /British blockade of its oil life line.