842 – The Oaths of Strasbourg
This famous peace treaty was made between Charles the Bald and Louis the German. What was at stake? A complicated family quarrel. The quarrel was caused in part by succession rules and the actions of their father, Louis the Pious.
In the oaths, Charles and Louis agreed to ally against their brother Lothar, the Holy Roman Emperor. What makes the oaths even more special, is their linguistic importance. The oaths are written in the oldest known French as well as Old High German (see also Britannica here and here).