Which war is "the war" in this context?

Poland and France are your prime candidates. Or Austria, even though it was annexed rather than occupied.

Poland had been divided between Germany, Russia and Austria for centuries, with a short period of independence since WWI.

France and Germany had been enemies for a long time. Especially the border regions of Alsac and Lorraine switched hands over the wars.

Austria and Germany fought over who would have the power in the old Holy Roman empire (or rather Prussia and Austria-Hungary), so there was a weird mix between belonging together and still feel different and competitive.

The kingdom of Serbia often had wars and disputes with Austrians and Germans since the late 1800's