You should look into it a bit more. The firebombing of Dresden was done by both the RAF and US bombers in February 1945. Over a three day period both forces bombarded the city as a whole, targeting civilians and not just military or industrial targets. And the firebombing of Japanese cities, not to mention Hiroshima and Nagasaki, were both indiscriminate and designed to kill as many civilians as possible. But that is war, and war is hard. So where were they any worse than Sherman was?
150 years after the fact...
Guernica in the Spanish Civil War, Dresden, Hiroshima, Nagasaki, Sherman's scorched-earth policy: although different in scope and in total impact, they had an intention in common: to target civilians areas deliberately, in order to shock the enemy into surrender. In this aspect --- the targeting of civilian areas as a means to an end --- these were not acts of legitimate warfare, soldier-against-miltary-targets, or soldier against soldier, but rather, war crimes.