Because Rhea was totally the instigator of that war, or even the one that escalated it to total destruction tactics first. /s
Instigator? No. However, Rhea's lack of supervision over Fodlan was but one of the causes that had led to the war. Rhea knew that those who had done harm to her people and Fodlan centuries ago, still existed plotting. However, rather than keeping vigilant watch, she instead focused primarily on reviving Sothis, which in turn led to many events that could've been prevented, such as the coup within the Empire that stripped Edelgard's father of his power, the Assassination of Faerghus's King, the Tragedy of Duscur, the experimentations on people to forcibly implant additional crests, such as Edelgard and Lysithea.
So no, Rhea wasn't the primary cause for the War; however it's not as though she was completely innocent.
Well, she did give order to have a Kingdom's capital set ablaze, which resulted in the deaths of many innocent civilians.
Only in one route, and only after going completely insane. The Rhea of Crimson Flower is very different from the Rhea of the other routes who never undergoes sanity slippage.
Instigator? No. However, Rhea's lack of supervision over Fodlan was but one of the causes that had led to the war. Rhea knew that those who had done harm to her people and Fodlan centuries ago, still existed plotting. However, rather than keeping vigilant watch, she instead focused primarily on reviving Sothis, which in turn led to many events that could've been prevented, such as the coup within the Empire that stripped Edelgard's father of his power, the Assassination of Faerghus's King, the Tragedy of Duscur, the experimentations on people to forcibly implant additional crests, such as Edelgard and Lysithea.
So no, Rhea wasn't the primary cause for the War; however it's not as though she was completely innocent.
Seteth notes in Silver Snow that they put exhaustive efforts into finding the people who had aided Nemesis in attaining power, but they were never able to find them. And given Rhea tried only 12 times to create a body for Sothis over the course of a 1,000 years I'd say she didn't focus primarily on reviving her.
The coup in the Empire, the Tragedy of Duscur, etc. are never indicated to have happened because Rhea wasn't paying attention, rather she and the church had no authority to intervene in the other nation's internal affairs like that (in particular the Empire had had rocky relations with them for some time). Indeed they only got to be so involved in Faerghus afterward because the central government was no longer able to handle things and requested their assistance.
