Dresden was payback for Coventry. Early in the war, Germany decided to destroy the harmless city of Coventry, England. While the Brits had already broken the Enigma code and knew full well what the German air force intended, they could do nothing about it without revealing that they had broken that key German code. Consequently, Coventry was destroyed. That was a war crime that had to be addressed. But in wartime, there are no courts of proper appeal (war is, in fact, the total absence of rules, law and judicial order). Payback was really the only way to address that crime. Payback was deserved; it was delivered. Period.
Among other things. Guernica was of no strategic importance. It was, pure and simple, a practice range for the Nazis. They were honing the Blitzkrieg.