A Hohenzollern (ruling house of Germany at the time) had been offered candidacy to a Spanish throne. France agitated that this should not happen, Prussia relented, than France pushed the issue even further saying that not only was it disallowed this time, but that it should be disallowed for any Hohenzollern to do this until the end of time. That was a bridge too far for Prussia - who refused - so France declared war.
The revisionism that somehow Bismarck tricked France into war with the Ems Dispatch comes from people trying to backport Hitlerism 60 years into the past.
This is called the Sonderweg theory . . . the idea that Germany somehow followed a unique historical path which none of the European powers did and which made Hitler's emergence more or less inevitable.
It's basically crap, as it blames the Germans for what happened even beyond the guilt which they should carry as the nation which elected Hitler into power. It denies the idea that such a dictator could dupe other nations into fascism, and that's both inaccurate and a dangerous assumption to make. It's basically a high-class academic version of "well, that could never happen HERE!"
The revisionism that somehow Bismarck tricked France into war with the Ems Dispatch comes from people trying to backport Hitlerism 60 years into the past.