Answer: Spain.
Explanation:
The Spanish Civil War was described as "dress rehearsal", because the bombing of Spanish cities by Nationalist rebels with the support of Nazi Germany and Fascist Italy, allowed them to test new weapons later used in World War II, like the He-111 and Do-17 bombers.
Particularly, gruesome was the bombing of Guernica, later depicted in Pablo Picasso´s painting because it was a military air force attack over civilians, which is considered as a war crime.
