Who knows how long its actually been around but the pot leaf can be traced back to the 1600s when Domestic production of hemp was encouraged. Farmers were required to grow it and according to the Pure Food and Drug Act it was ok to sell marijuana over the counter in foods. It wasn't until after the Mexican Revolution of 1910 when marijuana started gaining it's bad rep after immigrants introduced America to the recreational use of the plant. Unemployment increased during The Great Depression and government concerns began to grow which led to a whole lot of research that ended up claiming the use of the drug was linked with socially deviant behavior.
   By 1931 over 25 states outlawed marijuana. Eventually it was taxed but only accepted for certain authorized medical use. However it was still popular by the time the Vietnam war struck and many soldiers indulged in using. After all it was growing all over the place and was useful in keeping the soldiers relaxed during down time. Many known leaders overlooked this activity since it had fewer side effects than alcohol. 
Over time stricter laws were applied but this didn't stop the middle class civilians from taking part in the cannabis controversy. This caused a huge hippie movement where people were openly admitting to using and participating in parties where the drug was heavily influenced. 
   What was once known as a low class substance had now become considered a drug of enlightenment. 
Presidents Kennedy & Johnson claimed it to be a non violent drug and in fact stated it not to be a gateway drug at all. This claim led to reconsideration of penalties as to they were too harsh and non effective for such a misdemeanor.
Over time laws have been reduced and reinforced repeatedly.
Many group leaders and activist have fought for the freedom and restriction of the plant.
Workers have lost and quit jobs due to discrimination of the herb. There's been claims that the drug is pure evil and influences crime while others live by it's healing powers. Some may fear it's legalization may cause a communist war while some feel its a billion dollars industry that could open opportunities. Then of course you always got the big "Weed is safer than alcohol... and that's legal" argument. So who's to say what makes it wrong or right when the effects could vary among users?