Stepping into a region on edge, Vice President Joe Biden came to Poland on Tuesday to reassure anxious allies that the U.S. will stand up to Russia’s aggression in neighboring Ukraine, even as Moscow brushes aside sanctions and stern warnings from the West.

Biden arrived in Warsaw on Tuesday morning for consultations with Polish Prime Minister Donald Tusk and President Bronislaw Komorowski, a few hours after Russian President Vladimir Putin approved a draft bill for the annexation of Crimea, one of a flurry of steps to formally take over the Black Sea peninsula. Crimea on Sunday voted overwhelmingly to secede from Ukraine and try to join Russia.

In further meetings in the Polish capital and later in Lithuania’s capital, Vilnius, Biden was to discuss the crisis with the leaders of Estonia, Lithuania and Latvia — three Baltic nations that are deeply concerned about what Russia’s military intervention in Ukraine’s Crimean Peninsula might portend for the region. All four countries share borders with Russia; Poland also borders Ukraine.