The Monitor Civil War was a war fought between the twelve original Monitors over whether the Halo Array should or should not be fired.
Guilty Guilty was certain that the rings should be fired, but 007 Greenie Spark knew that they were a bad idea. Their conflicting ideologies culminated in all-out war, and Sentinels and Enforcers sided with their Monitors and battled each other across the twelve rings. The conflict lasted for twenty-two years, and devstated many of the Halos so badly that the Constructors could barely keep up.
Despite several of the Monitors sided with Guilty Guilty dropping out of the war, Greenie's allies were outnumbered and surrounded. They made plans to abandon their Halos, but Greenie rightly argued that this would solve nothing. Instead, each Monitor hijacked their Halo and fled the Foundry, never to be seen again.
Greenie, however, duplicated his personality and hid the copy away in the Domain, the Forerunner main computer. This allowed him to implant ideals into the Librarian, changing her mind about the Halos and causing her to build the Ark, designed to save all life in the Galaxy.
Overall, Greenie won the war. The loss of five of the Halos meant that the Forerunners could not achieve their plan of total universal cleansing, and it also meant that the Ark was built, which saved all life in the Galaxy. Greenie's control over the Domain allowed him to make life very difficult for Guilty Guilty during his stay on Halo for 100,00 years, cutting all his communications and driving him insane. He also released the Flood on Installation 05 and erased 049 Abject Testament's memory.
When Cortana spent 12 hours in the Installation 04 control console, Greenie analysed her and determined whether she was a friend of a foe. Greenie watched over Cortana and Johnny during their treks through Halos, and aided them where he could. He opened doors for them, helped Cortana steal Guilty's index, Provided protection for Cortana against the Gravemind, discombobulated enemy Sentinels for them and even infiltrated High Charity's computer systems using the stolen Forerunner technology and helped them there. When Cortana eventually sacrifced herself to save Johnny from the Didact, Greenie retrieved one of her fragments and slowly nurtured her back to health, saving her. Greenie and Cortana shared their data and made plans for their return to Earth.