KOLWEZI, Democratic Republic of Congo — Little about Kolwezi, a small city in southern Democratic Republic of Congo, hints at its global importance. Nondescript and ringed by slag heaps, pits and quarry lakes, the city is home to some of the largest copper and cobalt mines in the world. Now, Kolwezi is at the center of U.S.-Chinese competition over critical minerals. Earlier this year, mining firms began shipping ore along a U.S.-backed railway that terminates in Angola's Atlantic port of Lobito. A massive infrastructure project is focused on this rail line -- which is viewed as a bid to counter...