Oh, Silesia.
One of Frederick’s first acts upon becoming King in 1740 was to invade the Austrian province of Silesia, pretty much without any legal authority or rightful claim whatsoever. Part of it was his desire to cut a dashing figure on the international stage, part was his conception of the Romance of Battle (of which he quickly became disillusioned).
Mostly, I think it was Revenge. The Hapsburgs, rulers of Austria and virtually hereditary Emperors of the Holy Roman Empire, had spent a century raising the act of pooping on Prussia into a fine art. They carrot-and-sticked the hell out of Frederick’s father, borrowing his troops on promises of land grants that never came through, and then, on top of that, infiltrated his inner circle of friends with Austrian spies. Those spies, it turned out, were instrumental in preventing Frederick’s marriage to an English princess, forcing him to settle for a dull-wit favored by the Austrian house. All of this put together pissed Frederick off rather very much, and so when he got his chance, he took his army, stole Austria’s best land, and then kicked them in the junk every time they tried to get back up.
Oh, Silesia.
– DvL