Kriegsspiel (literally meaning "war-game") is a Prussian strategic tabletop game which was invented in 1811 by the fantastically-named Lieutenant Georg Leopold von Reisswitz. It's over two centuries old, and from a time when being a "wargamer" meant being born into nobility, having a glorious non-neck beard, and enlisting in some Officers Korps to train to become an armchair general.
In essence, it is the father of modern wargaming: two (teams of) players took control of opposing armies while a third neutral party intervened to simulate the inherent trouble of outdated intel, wrongly recieved and interpreted orders aznd the like. As a game, it's actually intricate and convoluted, but it was never meant to be played just for the lulz. Reisswitz intended his 'game' to sharpen the mind of upcoming officers for real tactics and what problems they might actually encounter in the field.
Reisswitz's ostentatiously named son: Lieutenant Georg Heinrich Leopold (or Rudolf?) Freiherrn von Reisswitz continued his fathers work and published an 1824 version, with a considerably expanded scope an ruleset.