"As William Butler once said: The Center cannot hold."
The First Insurrection was a galactic rebellion against the UEG, UNSC and CMA that started in the Outer Colonies, and later througout the entire Human empire. The War is technically a undeclared one.
It started when supplies for the Outer Colonies was starting to run out and the UNSC and CMA were pressuring them to join the UEG and give up their independence. This caused a set of riots and later the formation of the disorganized United Rebel Front. War was unofficially declared with the Callisto Incident.
The Insurrection started on October 9, 2494 and ended Feburary 17, 2554.