BANGKOK—With increasingly strong talk in support of Taiwan, a new deal to supply Australia with nuclear submarines, and the launch of a European strategy for greater engagement in the Indo-Pacific, the United States and its allies are becoming more assertive in their approach toward a rising China. China has bristled at the moves, and the growing tensions between Beijing and Washington prompted U.N. Secretary-General Antonio Guterres on the weekend to implore U.S. President Joe Biden and Chinese leader Xi Jinping to repair their “completely dysfunctional” relationship, warning they risk dividing the world. As the U.N. General Assembly opened Tuesday, both leaders chose calming language, with Biden insisting “we are not seeking a new Cold War or a world divided into rigid blocs,” and Xi telling the forum that “China has never, and will never invade or bully others or seek hegemony.” But the underlying issues have not changed, with China …