As Russian troops press ahead with a grinding campaign to seize eastern Ukraine, the nation’s ability to resist the onslaught depends more than ever on help from the United States and its allies — including a stealthy network of commandos and spies rushing to provide weapons, intelligence and training, according to U.S. and European officials. But even as the Biden administration has declared it will not deploy American troops to Ukraine, some C.I.A. personnel have continued to operate in the country secretly, mostly in the capital, Kyiv, directing much of the vast amounts of intelligence the United States is sharing...