system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is marijuana illegal in the first place?
gpt
It's best viewed in the context of the Temperance movement, which was a worldwide movement  against intoxicants which degrade man's ability to think and act rationally.  

The movement became politically influential in the 1860s, and the advent of WW1 created command and control economies with unprecedented governmental powers.  These powers combined with popular opinion created the regulation and outright banning of alcohol in many jurisdictions, particularly those influenced by Western thought, and other intoxicants were similarly judged and outlawed - any resistance to the loss of these recreational drugs were minimal compared to that of alcohol.  A second, and related movement crusaded to ban opium, probably the most banned drug in the world, which met with greater worldwide success than prohibition against alcohol. The emergence of highly addictive and cheap processed or synthetic drugs such as heroin or cocaine further encouraged a worldwide ban on certain drugs, which gave a legislative framework for both local, national, and worldwide illegalization of drugs.  Marijuana was banned at approximately the same time alcohol, opiates, narcotics and more were banned, and for the same reason.  Tobacco as well became strictly regulated although not altogether banned.

Dominance of Western nations ensured that other nations followed suit to bow to political pressure, as the drug trade was international.