By Simon Lewis SOUTHEASTERN POLAND (Reuters) -Washington’s top diplomat and its defense chief used the first official U.S. visit to Ukraine since Russia invaded in February to announce a gradual return of U.S. diplomats including a new ambassador and over $300 million in new military aid to Kyiv. Secretary of State Antony Blinken and Secretary of Defense Lloyd Austin traveled to Poland on Saturday in a previously undisclosed trip, and took a train the following day into Ukraine where they met President Volodomyr Zelenskiy and other top Ukrainian officials. “In terms of Russia’s war aims, Russi…