Heads of state are meeting in Warsaw for a two-day NATO summit that U.S. President Barack Obama said this “may be the most important moment for our transatlantic alliance since the end of the Cold War.” Much of the focus is on Russia, which seized the Crimean Peninsula from Ukraine in March 2014 and backs separatists whose war with Kyiv’s forces has killed more than 9,300 people in Ukraine's eastern Donbas region. “Russia’s aggression against Ukraine threatens our vision of a Europe that is whole, free, and at peace,” Obama wrote in a commentary published on the Financial Times website shortly before the summit began. “I believe that our nations must summon the political will, and make concrete commitments, to meet these urgent challenges,” wrote Obama, who also met with EU leaders. Russia’s interference in Ukraine has increased concerns in eastern NATO nations including Poland and the Baltic states of Estonia, Latvia, and Lithuania, which were under Moscow’s thumb until the collapse of communism and the disintegration of the Soviet Union a quarter-century ago. The leaders of Finland and Sweden are sitting beside their NATO counterparts at an alliance summit for the first time, reflecting growing cooperation amid an increase in Russian military activity near their borders.