
U.S. Secretary of Defense James Mattis (R) and Ukrainian President Petro Poroshenko walk to the Pentagon in Washington D.C., the United States, on June 20, 2017. (Xinhua/Ting Shen)

U.S. Secretary of Defense James Mattis (R) and Ukrainian President Petro Poroshenko walk to the Pentagon in Washington D.C., the United States, on June 20, 2017. (Xinhua/Ting Shen)