Godzilla: King of the Monsters reveals that long ago, humanity engaged in a costly war with Titans. It would seem that the history between Man and Titan in the MonsterVerse was more complicated than it was originally thought. Also, it appears the humans and not the monsters were the villains of the story.
The third film in the MonsterVerse touches on the history of the Titans and their association with humans when Monarch goes down into the Hollow Earth tunnels to find Godzilla. They discover an ancient underwater civilization that was apparently Godzilla's home. Paintings on the walls indicate that the humans who once lived there worshiped Titans like Godzilla as gods. It's suggested that back then, humans revered the Titans and treated them with respect rather than fear and suspicion, which is how modern humans tend to regard the monsters.
The redacted text in the credits of Godzilla: King of the Monsters tell a very long story about the relationship between humanity and Titans, which goes back thousands of years. At the beginning, humans did indeed worship Titans, but things changed when humans somehow developed a method of telepathic communication with the Titans. These ancient humans sought to gain control over the Titans, who they believed could be used for purposes of warfare. The Titans became slaves to the humans, but some were able to rebel against their masters. As a result, a war ignited between them.
Though the credits don't reveal a winner, they do say that the human survivors scattered and formed colonies all over the world, which does hint at defeat for mankind. Also, whatever happened in the war caused an event that the redacted text refers to as "the cataclysm". It's not clear what this "cataclysm" was, but it was somehow devastating enough to trigger the Ice Age, which means that the world's biggest weather phenomenon is related to the actions of Titans (and humans) in the continuity of the MonsterVerse.
This would mean that the war is the single most important event in the history of humanity. The war reshaped human civilization by scattering the survivors all over the planet, forcing them all to start over. The results of the war can serve as a warning for modern humans in the MonsterVerse, since Godzilla: King of the Monsters reminded humans of the existence of Titans and just how much destruction they can cause if provoked.