The fact that this undoubtedly violates international law means nothing. Nonetheless, I expect the courts to strike this down. Not that Trump can’t do, he just can’t do it by executive order. Under current law, you can be denied asylum if you are found to be “firmly resettled” in a third country. See 8 CFR §208.15 for the definition of firm resettlement. The administration could change this regulation but to do so, they would have to use the procedures set forth in the Administrative Procedure Act. Alternatively, Congress could change the law by legislation. Since this is unlikely to hold up in court, I take it that this is just another piece of red meat thrown to Trump’s base to keep them happy.