The War of Independence
The League of Independent Systems and Planets (LISP) arose to oppose the Alliance unstoppable spread. The war lasted 6 years, and had many heroes on both sides.
Ultimately, the LISP was unable to match the industrial might of the Alliance and crumbled under the constant attacks. The in-fighting between systems and internal politics of LISP leadership ensured that they would never be able to match the Alliance on the battlefields of space.
That war ended a decade ago, and there still exists vast fields of destroyed vessels, and ruined battlefields spread out across hundreds of planets. Many of the commanders of LISP battle fleets when the war ended went AWOL to become pirates or independent mercenaries for hire. For them, the alliance will always be an enemy.