Throughout the early years of the war, [Reich] officials tried to provide a legalistic image, often by shifting blame for the onset of terror bombing to the British. In May 1940, the New York Times published a column claiming that Hermann Göring, the supreme commander of the Luftwaffe, threatened retaliatory strikes against Great Britain, specifically using the word terror in its inflammatory language.²¹ Promising “mighty blows” against the British body and flexing the perceived muscle of the [Luftwaffe], Göring was laying blame for his intentional targeting of civilian populations squarely on British military operations against Germany.²²

While this was framed as a retaliation for British operations and the Blitz—an all-out aerial assault aimed primarily against civilians which lasted almost a full year—is often cited as the beginning of German aggression towards civilian populations, that explanation is far from the truth. It is important to remember that [Fascist] fire rained on Warsaw and other Polish cities months before the publication of this article, not to mention Guernica years before.

While Göring threatened only Great Britain, the Luftwaffe went on to target Dutch and Belgian cities as well as French border towns like Mareuil with terror strikes only five months later, using the same methods that were tested at Guernica. The first sorties of the Blitz didn’t fly until early July of 1940, long after the Reichsmarschall made his threat.

(Source.)