MUNICH, Germany — German Chancellor Friedrich Merz said that U.S. leadership can no longer be taken for granted and Europe must prepare to stand more firmly on its own, in a stark warning to world leaders in Munich. “The leadership claim of the U.S. is being challenged, perhaps already lost,” Merz said during the opening of the Munich Security Conference , laying out the starkest assessment yet from Berlin of a world increasingly defined by great-power rivalry. “In the era of great powers, our