It's difficult to pick the most destructive of America's many senseless, futile and tragically needless wars, but the "War on Drugs" is near the top of the list. Prohibition of mind-altering substances has not just failed--it has failed spectacularly, and generated extremely destructive and counterproductive consequences.
What was the result of the Prohibition of alcohol in the 1920s? Prohibition instantly criminalized 40+% of the adult populace and created hugely profitable criminal organizations.
What was the result of the "War on Drugs"? This modern-day Prohibition instantly criminalized large swaths of the adult populace and created hugely profitable criminal organizations.