Changing of the Guard

"This is unprecedented. Unacceptable."

- Special Operations Commander Rtas 'Vadumee

"Recommissioning the Guard was a radical step. But recent events have made it abundantly clear that the Elites can no longer guarantee our safety."

- High Prophet of Truth

Though there were many causes for the Great Schism, some old and some recent, the Changing of the Guard was the final spark that ignited the conflict.

History
After the assassination of the High Prophet of Regret by John-117, the remaining two Hierarchs claimed that the Sangheili were no longer capable of protecting the Hierarchs and subsequently reassigned the Sangheili Honor Guards, replacing them instead with Jiralhanae. This was in flagrant violation of the Writ of Union, the founding document of the Covenant, and was a direct insult to the Sangheili as a people, who threatened to resign from the Covenant. In response, the Prophets declared the Elites to be traitors to the Covenant, and encouraged the other races to turn against their former commanders and remain loyal to the Prophets, igniting the Civil War.

However, the High Prophets had not foreseen the support the Sangheili still had among much of the Covenant. Many Unggoy and Mgalekgolo broke away with the Sangheili, joining the Separatists, and the conflict between them and the Loyalists began the Great Schism.

Trivia

 * The High Prophet of Truth took a dismissive approach to the early rumblings of discontent, deeming the event merely an "exchange of hats".
 * "Changing of the Guard" is a phrase used in modern-day Britain, signifying the changing between different shifts of guards who protect the royal family.