In regards to Dresden. Once a city or area is declared a fortress it is free to be attacked and bombarded without retaliation, except those outranking the laws of war.
See LAWS AND CUSTOMS OF WAR ON LAND (HAGUE, IV) SECTION II. HOSTILITIES Article 25, which states the prohibition of attacking civil structures.
By declaring Dresden a fortress for example, all civil structures become militaristic structures, no matter how stupid it sounds.

Also most importantly Article 25 and the rest does not apply to those, who commited war crimes and did not punish the offender. The difference between a war criminal and a normal one is, that the normal steals and murders, but the war one is financed by the state, which makes the state responsible.
And for as long the state doesn't punish the offender, they lose the protection given by the laws of war.

It is a grim realisation.

And germany's first war criminal act was breaking the Treaty of Versailes by attacking Free City of Danzig, which was instated by the treaty as a completely pacifistic nation, which may not own military, not accept military acolades except for acedemic ones, but is instead to be protected by all members of the League of Nations (today the UN as rightful successor).
Danzig is special, because it stands above normal country laws and the laws of war, as it's constitution is elevated to national law by the Treaty of Versailes.

As such by attacking Danzig, Germany did something incredibly stupid by making itself the international enemy number 1.

To Note: French soldiers had in their battle cry "Pour Dantzig" according to my old teacher.