The states entered into a union for the purpose of protecting themselves from foreign invasions and Section IV of Article IV made sure the government knew that was their department. The only part of Article IV that I found surprising was Section III with the addition of the new states, I didn't know Congress had the power to accept new states and that interested me. I found that Article IV was a general overview of the states where the founders sought not to limit the states' actions but still ensure peace among them.