Every April 20, marijuana smokers around the country light up for an unofficial holiday celebrating pot that stems from the smoker slang "420." This year, as the drug war rages in Mexico, the festivities fall against an increasingly violent backdrop.

Some antidrug advocates are using the occasion to jump-start a movement against marijuana not just for health and legal reasons, but on moral grounds. American pot smokers, they say, are unwittingly supporting drug cartels in Mexico.

Aaron...