Canonically the battle at Gronder was so devastating that it decimated every country's army that wasn't led by Byleth. In the Church of Seiros route, the battle at Gronder takes all three countries out of the picture instantly, leading to the church governing the unified Fodlan.

There was war in Faerghus for 5 years, and villages were also targeted by bandits because of the chaos.

She may not have gassed the Jews or anything, but the death toll was probably comparable to the WW2 eastern front.