My mom was born in 1917, and grew up believing that, when the Germans marched into Belgium in 1914, they attacked civilians, and skewered babies with their bayonets.
That was British propaganda. But it went a long way towards selling the war to the U.S.
After all, that same propaganda had gone a long way towards selling the war to the British, the Australians, the Canadians etc.
The end result was a disaster.
(By the way, as of the 1970s, mom was aware that she was misinformed with regards to the Germans.)