What event causes the ants to begin to prepare for battle in the once and future king?

In "The Once and Future King" by T.H. White, the ants begin to prepare for battle after the arrival of a rival colony. The ants are driven by their instinctual response to threats and their need to protect their territory. This preparation for conflict highlights the themes of duty and the nature of war, contrasting with the more nuanced views on conflict that King Arthur embodies. The event serves as a pivotal moment for the protagonist, Wart, as he observes the rigid and often brutal society of the ants.