Enduring characteristic of international relations is "anarchy"

Why does the instructor say that an enduring characteristic of international relations is "anarchy"?
Why was Thucydides considered the first "Realist" in International Politics?

1. If those (states) with power can do as they choose and those (states) with little or no power must submit is
   there anything more than power operating in international relations?
2. How do states mitigate anarchy in International Relations