A few weeks ago, rare earth elements, critical minerals, and other natural resources seemed like just one more factor in efforts to end Russia's war on Ukraine. Now they are front and center, a dominant focus of U.S. talks with Kyiv and a prominent vein of the intensifying diplomacy that also involves Moscow, the European Union, and others as Russia's full-scale invasion enters its fourth year. U.S. President Donald Trump said late on February 25 that Ukrainian President Volodymyr Zelenskyy wants to come to Washington on Friday to sign a "very…