Oman, a sultanate long prized for its diplomatic neutrality and its willingness to serve as a quiet bridge between Washington and Tehran, found itself this week on the receiving end of a military threat from Donald Trump — the very power that had asked it to mediate. The threat arrived without explanation, leaving Omani officials to search for what they might have done to deserve it. In threatening a country that has functioned as an instrument of American diplomacy, the United States risks dismantling the very architecture of back-channel communication it has relied upon for decades.