"Merely secession" doesn't give it the weight it needs. The Outer Colonies wanted out from under the heel of Earth and the Inner Colonies as they weren't getting the resources and amenities the rest were getting. The Outer Colonies' raw materials and immense food provisions, among other necessities, were absolutely needed for the very survival of Earth and the Inner Colonies. I wasn't like they could just be set free to govern themselves. That's why ending the Insurrection was so vital.