My response to Morse's clear explanation of why law enforcement can neither cut supply nor demand in a drug war: Take drugs out of the realm of law enforcement. If drugs are decriminalized they will no longer be inordinately profitable to growers and traders. Their lure for the counterculture will subside.
If currently illegal substances become legal, moreover, their purity can be "policed" by the Food and Drug Administration, thereby averting many deaths caused by street supplies.
Remember that bootleggers were the strongest (and richest) advocates of Prohibition. The fruitless war against drugs similarly is a bonanza to dealers and suppliers. Decriminalization wouldn't be popular with them, and they'd make sure the campaign to maintain current laws would be spearheaded by holier-than-thou anti-drug crusaders. But the big money will come, sure enough, from those who profit most from the ever-doomed efforts to apply the law.