the start of the prohibition on hemp/cannabis and other mind altering chemicals was due to corporate influence into a corrupt legislator/goverment at the time.
dupont [ http://www.corporatewatch.org.uk/?lid=202
they had interests in synthetic textiles around the time of the ban on hemp was created/enforced
when nixon took office in late 60s and then created the controlled substances act of 1971 i believe the motive behind this was due to the fact that there was an increase in lsd use in the 60s and a whole new generation of anti-vietnam "free thinkers" had been born. this was unacceptable for the goverment of the time.
the current state of war on drugs is kept alive by the huge amount of funding that is created by it, as well as complete and utter brainwashing that took place a few decades ago and now the people who are in power come from this era. Once somone has been indoctrinated to such a point, even after showing them real evidence, photos, documents, case studies, they will still not be able to break free of their indoctrination.
this just some and ofcourse not all the reasons why the drug war exists, and just my opinion after what ive read and watched etc...