CARDIFF, WALES — President Barack Obama accused Russia of fomenting violence in Ukraine and told the Baltic states that NATO and the United States military will respond if Russia attacks a member of the alliance.Speaking in Estonia before traveling to a NATO summit, Obama pledged that the three countries independence will always be guaranteed by the strongest military alliance the world has ever known.He said the former Soviet Republics had successfully embraced democracy, but that Russias aggression against Ukraine threatens that progress.Russia has denied its provocations in Ukraine, but Obama said the destabilization there was caused by pro-Russian separatists who are encouraged by Russia, financed by Russia, trained by Russia, supplied by Russia and armed by Russia.And the Russian forces that have now moved into Ukraine are not on a humanitarian or peacekeeping mission, he said of a recent convoy of trucks that the Russians say carried humanitarian supplies. They are Russian combat forces with Russian weapons in Russian tanks.He pledged NATO and US support toward fending off more incursions, saying more NATO aircraft now patrol the Baltic skies and more American forces are on the groundtraining and rotating through each of the Baltic states.And he said he supports NATO extending what he called defensive measures for as long as necessary. Article 5 is crystal clearan attack on one is an attack on all, Obama said, referring to the NATO defense provision. Well be here for Estonia. Well be here for Latvia. Well be here for Lithuania. You lost your independence once before. With NATO, youll never lose it again.He said the U.S. is working to bolster NATO security and further increase Americas military presence in Europe, including positioning more American equipment in the area and mean more training and exercises between militaries. It would also mean more U.S. forcesincluding American boots on the ground continuously rotating through Estonia and Latvia and Lithuania, he said.He also renewed a call for more NATO members to boost their defense spending to at least 2 percent of their GDP. He singled out Estonia, which already does and said Latvia and Lithuania have pledged to do the same. This weeks summit is the moment for every NATO nation to step up and commit to meeting its responsibilities to our alliance, Obama said.And he called on NATO and allies to stand against Russian President Vladimir Putin, whom he said has repeatedly declined to resolve the Ukraine crisis diplomatically.