The United States could have entirely eradicated it's drug problem during World War 2 because our attention was focused on defeating the Axis Forces. But we had already dug a hole for ourselves by entering into an agreement with the Mafia.
During World War 2, to secure Italy, the United States Government formed an alliance with the Mob -- specifically Lucky Luciano -- who was imprisoned in New York. Since Mussolini didn't hold the La Cosa Nostra in high esteem and tried to break them, Luciano turned to the American authorities and enabled us to ultimately dock and take over Italy during the war. As payment for his services, Luciano was released from prison and deported back to Italy after the war, where he revived the Mafia and established links in the United States.
This fact is very important, for after Prohibition was abolished, the Mob began trafficking drugs, which had a much higher profit margin than alcohol.