After escaping the monastery and witnessing Rhea's transformation into The Immaculate One, Krynn's Vanguard retreated with Edelgard and the Imperial Army to Landfall where they were regrouping and planning their next steps.
Edelgard gave an impowering speech to those present at Landfall and officially declared war on the Church of Seiros. After some talking it was clear that Edelgard's intentions are to take down the Church of Seiros and create a meritocracy that does not take into account one's lineage nor whether they have a mark or not. A societal system created by the church. This would also involve reunifying the Kingdom and Alliance back into the Empire as they originally were.
After a couple weeks with the Imperial Army recouping and waiting on the main army coming from the Imperial Capital, the army and Krynn's Vanguard set out from Landfall to attack and conquer Garreg Mach. It would be a strategic and symbolic attack. It would grant a significant advantage on taking control over Northern Ergoth and also pushing the Church out, particularly pushing them to retreat to the Empire as Manic and Hylo are not particularly fond of getting involved in others conflicts.