By: Shim Jae HoonFrom security ties to trade and business relations, the US-South Korean alliance, long hailed as iron-clad and most successful in East Asia for decades, is coming under exceptional stress, threatening the whole fabric of their long-term relations. In a worrisome sign that surfaced on July 30, the newly arriving US ambassador to Seoul, Michelle Steel, a Korean-born former Republican congresswoman from California, was greeted at I…