Obama also offered high-profile gestures of American support for Ukraine, holding talks for the first time with the former Soviet republic's new president-elect and announcing long-sought assistance for the Ukrainian military, including body armor and night vision goggles.
"The people of Ukraine are reaching out for the same freedom and opportunities and progress that we celebrate here today — and they deserve them, too," Obama declared in a rousing address to 6,000 people gathered in an outdoor square in the heart of Warsaw.
The celebration marking the 25th anniversary of Poland's first partially free election was the centerpiece of Obama's two-day visit to Warsaw.
Shortly after his remarks, Obama traveled to Brussels for a meeting of the Group of Seven major industrial nations, where leaders were expected to issue stern warnings to Russia but hold off on imposing further economic sanctions.
Obama will close his trip in France on Friday, marking the 70th anniversary of the D-Day invasion during World War II. At the ceremony, Obama probably will have his first face-to-face interaction with Russian President Vladimir Putin since the crisis in Ukraine unfolded earlier this year.
Putin's annexation of the Crimean Peninsula from Ukraine as well as other threatening moves have rattled Poland and its neighbors, many of which have historic ties to Russia, but also deepening relationships with the West.
Seeking to offer assurances to those nervous nations, Obama vowed to come to the defense of Poland and other NATO allies, declaring that an attack on one member country would be an attack on them all. He also reaffirmed his commitment to seeking congressional support for up to $1 billion for a "European Reassurance Initiative" that would boost the American military presence on the continent.
"After investing so much blood and treasure to bring Europe together, we refuse to allow the dark tactics of the 20th century to define the 21st," he said.
Earlier Wednesday, Obama met for the first time with Petro Poroshenko, the billionaire candy maker who handily won Ukraine's presidential election last month. The talks were an attempt to bolster the legitimacy of Ukraine's new government, which Russia is yet to recognize.
Poroshenko will be inaugurated on Saturday with Vice President Joe Biden and other Western dignitaries on hand. Sitting side-by-side with Obama, Poroshenko thanked the U.S. for its support for "the Ukrainian people, for freedom, for democracy."
While Ukraine's successful elections were an important milestone, Poroshenko faces enormous challenges stabilizing his country's economy, ensuring a steady flow of energy supplies and tamping down violence stirred up by pro-Russian separatists who have occupied eastern cities.
Obama's critics contend he has failed to provide the Ukrainian military with the assistance it needs to push back against those insurgents. Up until now, U.S. assistance has been largely limited to clothes, food and radios.
The new $5 million assistance package is relatively modest and is unlikely to significantly change the trajectory of the conflict in eastern Ukraine. White House officials said Poroshenko pressed Obama for even more military help, though they said the Ukrainian leader stopped short of asking for lethal aid.
White House deputy national security adviser Ben Rhodes said the president was "very open" to discussions on more ways to bolster Ukraine's military but continues to be focused on non-lethal assistance.
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