China

But first I'd expect more fearmongering and a build-up. Maybe they'll send in some CIA patsies, get them caught doing the worst attempt at spying in the world (I suspect they may have done this in Russia) but one that can be charged in the Chinese justice system and one which is plausible enough it may be construed as a real attempt at spying if a relatively low impact one. Fear-monger and saber rattle over it, accuse China of kidnapping and arbitrary detention, blast it all over the news, put it in popular media, make the American people afraid that if they go there they'll be arrested and imprisoned "just for being American", ramp up accusations China will spy on your phone, that just bringing your phone there will allow the evil Chinese cell networks to implant see see pee spyware on your phone.