The Monster Civil War was a war fought between the Kingdom of the Monsters and the Confederacy of the Monsters that lasted from August 26, 1871 to May 9, 1875. The war started over the policy on Humans that fall into the Underground needing to be changed. When confronted with this, Asgore Dreemurr refused to change his position, forcing Grillby Vulcanus and Gerson Kachhua to succeed from the Kingdom of the Monsters, and form the Confederacy of the Monsters. Asgore, infuriated by this betrayal, declared war on the Confederacy. This action wasn't particularly supported within the Capital, so Asgore simply stayed back, building an army, looking for any excuse he could to get the people on his side for an invasion of the Confederacy. This reason was eventually found when it was discovered that the Confederacy was sheltering a Human. With this knowledge, the people were in full support of the invasion, and within days, the Loyalist army was crossing the border at Waterfall Caves. In response, Gerson collapsed large sections of the cavern, cutting off all communications. This prompted Asgore to send a majority of the Loyalist army through The City of New Home to reach the Snowdin Province. During this massive re-mobilization, Grillby rallied all the Monsters that were willing to fight and die for the protection of the Justice Human, and marched to meet the Loyalist army in the Lower Snowdin Province. It was there that the first true battle of the civil war happened.
More to come.