I would be very clear, not “clear” like Mr. Let Me Be Clear While I Lie Through My Teeth To You, but truly clear, especially to other nations.
To North Korea I would say, this. The next threat you issue towards us, or our allies will be taken seriously, and will result in your country losing a major military installation of our choosing. A second threat will result in, well, you might not want to even think about that.
To Hamid Karzai I would say. We are leaving, you are on your own, you will not place our military at a disadvantage with moronic rules of engagement that favor terrorists.
To Mexico I would say simply that we are going to seal our border and control who enters. Any group caught smuggling drugs into America will be found and eradicated by any means within our power to bring to bear. Also those drug cartels, which pose, in my view, a threat to Americans living in border states, will be dealt with by YOU, or I will confer with my military leaders and discern the most effective way to eradicate that vermin. And we will do so, and you will stay the Hell out-of-the-way.
To Iran a simple note would suffice. If you continue pursuing nuclear weapons, and thus posing a grave threat to us and our allies, I will not only support Israel in taking your capability out, but the United States military will be taking full part.
To China just this. Dear Chicoms, we will fly our fucking planes where we want WHEN we want, end of discussion! Oh, and about Taiwan, and Japan, do not even think about it.
Finally to the UN just a few words. Get you building the Hell out of America, YESTERDAY!