Ways World War I Screwed Up the World

Just before WWI:

We lived through an abrupt shift from agriculture to industry: At the time of the US Civil War (1860), 80% of Americans labored to make our food; by WWI, 55 years later, only 20% did.
Major nation-states formed up.
Railroads, then roads, criss-crossed the nation. Cars filled the streets. "Peak horse" was around 1915.
The telegraph and photography transformed journalism forever, multiplying the speed and impact of news. Before photography, battle painters often made war seem quick and glamorous. Mathew Brady's photos of bloated corpses left at Antietam shocked the public.
The Scramble for Africa which ended with WWI, really messed up Africa, compounding slavery's terrible effects. See especially King Leopold II of Belgium's awful Congo Free State.

During and right after WWI:

Revolutions were in the air, especially Communism. Took Russia out of the War. Scared everyone else.
The Sykes–Picot Agreement (1916) screwed up the Middle East
Infamous Versailles Treaty (and Britain's weird role)
Eddie Bernays, the 1919 Paris Peace Conference, and his uncle Sigmund. Propaganda becomes PR.
Ho Chi Minh wrote Woodrow Wilson to ask his assistance in creating a democratic Vietnam
Machine guns, high explosives, poison gas...
The meat grinder
The end of gentlemen soldiers
Shell shock
Tanks and military aircraft
The Spanish Flu started at WWI's end, and killed more people. It also shredded the social fabric. From Laura Spinney's book Pale Rider: "Your best chance of survival was to be utterly selfish... jealously guard your hoard of food and water, and ignore all pleas for help." The Spanish Flu was so awful that nobody wanted to talk about it afterward.