In the quiet corridors of factories in Xian, Dalian, and Wuxi, two of the world's most powerful chipmakers have been conducting an experiment that speaks to the fragility of technological alliances in an age of geopolitical rivalry. Samsung and SK Hynix, long dependent on American equipment to sustain their Chinese operations, have spent two years testing Chinese-made alternatives — not out of preference, but out of prudence. The United States, in its effort to contain China's semiconductor rise, has inadvertently handed Chinese equipment makers their most consequential audition yet, and the o