In 1410 Grand Duke Vytautas of Lithuania was after the Teutonic Knights and thought they were smuggling weapons on trade ships. Vytautas ordered some Lithuanians (Samogitians) to storm the Teutonic city of Königsberg and "drown the knights"

It is said Vytautas boasted that he would "water his horses in the Rhine".

My question is regarding the use of the word "his". Is Vytautas referring to watering 'his own horses', or is "his" referring to the horses belonging to the Teutonic Knights?

If it is the horses of the Knights: By "watering" does he mean 'drowning' or 'claim their horses as their own be giving them water from the Rhine'? And perhaps, what is the significance of the Rhine river?