Beitzah 25 proves from a piece of raw meat from a hospitalized cow that we are not allowed to eat at the end of yom tov – that derech eretz is from the Torah and, in general, Hashem gave us Torah so that oterwise, Jews with their azut will not give space to other nations to live.
A fitting food for thought for Simchat Torah and a l’chaim for an increasing derech eretz on this site.