There were 3 major Wars.

War SIGMA: This was the war after the new multiverse was created, between Perisma and the ancestors. Happened about a Eon before the events of chapter 0

War BETA: This was the War that sent all monsters into the underground approximately 100 years ago

Major event:The first fallen.

About 20 years before the events of chapter 0, Chara fell into the underground to be greeted by Asgore. She (Chara) was introduced to Toriel and Asriel soon after. About a year later a second War had began (War Alpha), a soldier had fallen and in a state of panic attacked multiple monsters of the Ruins. Only a few were killed, but Asriel was one of them. The new family were distraught after this event. Toriel decided it was best to create a new home on the other side of the underground for safety reasons. Nabstablook had volunteered to look over the ruins since he was a ghost and unkillable. Asgore went into research with alphys and the famous Doctor Gaster to find a way to make a artificial monster soul. Over the next 5 years, 6 humans fell into the underground. Asgore had taken their souls and successfully made a artificial soul.

War ALPHA: A War between nations and divisions had broken out causing a human to search for refuge. Happend 10 years before chapter 0

Major event: The seventh fallen.

Frisk had looked into history and had heard that many disappearances had happened near MT. Ebbot. Both curious and hoping to get away from the War, they had wandered through and like many before, fell into the underground. The trip was much like the base UT story, but said they could not leave until another human falls and tells them ot is safe. Little did they know the next human would release them, through death and destruction, to start off the end times.

The Eighth fallen will be discussed later