Wild guess right out of the blue as to how that could happen: Impeachment over some issue or other (business deals with China, lying about who knows how much, etc) or being removed due to being unfit for duty (is that the 25th amendment?). The intertextual message to other heads of state, presidents and so on being: "Look. We can depose the US president. What chance do /you/ think you have in your pesky little nation?" But as I said, I'm just firing blindly into the night so to speak.