U.S. President Barack Obama is welcomed by Prime Minister Donald Tusk (L) at the Prime Minister's Office in Warsaw June 3, 2014. Obama is visiting Warsaw, Brussels, Paris and Normandy this week where he is expected to elaborate on the U.S. commitment to counter Russian moves against Ukraine and reassure nervous allies the United States has their backs. REUTERS/Kacper Pempel (POLAND - Tags: POLITICS)