How does the bugger war happen - enders game?

Answer 1

The Buggers fly in from space and start attacking the humans

Answer 2

In "Ender's Game," the bugger war is a conflict between humanity and an alien insectoid species called the Formics. The war began when the Formics attacked Earth, resulting in heavy casualties. To prevent the Formics from launching another invasion, humanity launched a preemptive strike on the bugger homeworld, ultimately leading to their near extinction.