BRUSSELS -- President Barack Obama has spent much of his presidency mired in the challenges of a world well outside the borders of Europe -- the turmoil of the Middle East, the power struggles in Asia and the terrorist threats percolating in northern Africa, Pakistan and elsewhere.
But Russia's invasion of Ukraine and its stunningly fast annexation of Crimea have demanded a renewed focus on a part of the world that dominated U.S. attention in the 20th century.
Often accused of neglecting Europe in his first five years in office, Mr. Obama is using this trip to recommit to the NATO alliance, rally the continent against Russian "brute force" and cast the showdown as a test of common values.
To show resolve, Mr. Obama decided on Wednesday to modestly increase military deployments in Eastern Europe and aides said he would intensify efforts to broaden energy security, negotiate a trade agreement with Europe and upgrade military capabilities.
Yet it will be hard to back up words with resources. The U.S. has only a fraction of the force it once had in Europe, expanded energy ties will take years and his own party leaders oppose quick action on a new trade pact.
During the height of the Cold War, U.S. troops in Europe numbered around 400,000, a combat-ready force designed to quickly deploy and defend Western Europe -- particularly what was then West Germany -- against a potential Soviet advance.
Today there are about 67,000 U.S. troops in Europe, including 40,000 in Germany, with the rest scattered mostly in Italy and Britain. The Air Force has some 130 fighter jets, 12 refueling planes and 30 cargo aircraft. At the end of the Cold War in the early 1990s, it had 800 aircraft in Europe.
The U.S. Navy, meanwhile, has dropped to 7,000 sailors and Marines, down from the 40,000 sailors who were stationed at nine major Navy bases during the height of the Cold War. Today, there are no U.S. aircraft carrier groups based in the Mediterranean, although the Navy does have one destroyer deployed at Cádiz, Spain.
Pentagon officials will not make public precise details about the U.S. arsenal of weapons and equipment in Europe for security reasons. But an official with European Command, which is responsible for U.S. military operations in Europe, said Wednesday that the U.S. military presence there was 85 percent smaller than it was in 1989.
Moreover, Mr. Obama next month will head back to Asia, where aides said he will again promote his policy of pivoting toward the region he believes represents the future. One goal then for Mr. Obama, aides said, is to challenge Europe to take more of a leadership role itself, a familiar theme from Washington but one infused with new urgency by the Ukraine crisis.
"Russia's leadership is challenging truths that only a few weeks ago seemed self-evident," Mr. Obama said in a speech here on Wednesday. "That in the 21st century, the borders of Europe cannot be redrawn with force; that international law matters; and that people and nations can make their own decisions about their future."
"The contest of ideas," he added, "continues."
Julianne Smith, a former national security aide in Mr. Obama's White House now at the Center for a New American Security, said she did not expect "a fundamental recalibration of U.S. foreign policy" toward Europe, but a more "symbolic" shift.
Yet if the renewed U.S. attention to Europe proves largely symbolic, that by itself might represent an important shift. European leaders and analysts have long complained of Mr. Obama's lack of interest.
But Russia's actions in Ukraine appear to have sent an electric jolt through Mr. Obama and his team.
At first, when a political crisis broke out in Kiev in November, he largely left the matter to Vice President Joe Biden. Since President Vladimir V. Putin ordered troops to seize Crimea, Mr. Obama has become increasingly engaged, blitzing foreign leaders with telephone calls, imposing sanctions and speaking out more frequently.
To reassure nervous allies, he sent six extra F-15C Eagles to Lithuania and 12 F-16 fighter jets to Poland.
Mr. Obama, who met here with Anders Fogh Rasmussen, the NATO secretary-general, will further bolster defenses in Eastern Europe by rotating more ground and naval forces for exercises and training in Poland and the Baltic countries; updating contingency planning; and increasing the capacity of a NATO quick-response force.
"Putin just declared war on the European order and it's demanding that the United States focus on Europe again as a security issue," said Damon Wilson, a former national security aide to President George W. Bush and now executive vice president of the Atlantic Council.
While some Republicans have pushed the president to be tougher, Mr. Wilson praised Mr. Obama's response. "I don't think I've seen the president more personally engaged on any foreign policy crisis in a concerted way as he has been on Ukraine."
Mr. Obama's speech here on Wednesday offered his most extensive rejoinder yet to Russia, a point-by-point refutation of every justification offered by Moscow for its seizure of Crimea. He rejected Mr. Putin's comparison of his actions in Crimea to the West's actions in Kosovo. He said Kremlin claims of persecution of Russian speakers in Ukraine were not true and called it "absurd" to suggest America supported fascists in Kiev.
"No amount of propaganda can make right something that the world knows is wrong," he said.
Perhaps most strikingly for a Democrat who rose to prominence in part due to his opposition to the Iraq War, Mr. Obama took on and dismissed the Russian claim that the U.S. was hypocritical because it used force to dislodge Saddam Hussein. He reminded his audience that he spoke out against Mr. Bush's decision to invade in 2003.
"But even in Iraq, America sought to work within the international system," he said. "We did not claim or annex Iraq's territory. We did not grab its resources for our own gain."
Mr. Obama also pivoted away from his own rhetoric in 2012 when he mocked Mitt Romney's us-against-them assessment of Mr. Putin by suggesting it was old-world thinking at a time when history had largely moved on. Now, it was Mr. Obama who evoked history, visiting Flanders Field Cemetery, a burial ground in Belgium for thousands of Americans who died in World War I, and later recounting the "ongoing clash" of two sets of national ideas about power and values during the Cold War.
"For decades, this vision stood in sharp contrast to life on the other side of an Iron Curtain," Mr. Obama said, describing democratic ideals fostered by European countries. "For decades, a contest was waged, and ultimately that contest was won."
In his speech, the president called for a reaffirmation of Atlantic relationships, suggesting the Europeans and Americans had become complacent since the collapse of the Soviet Union. He said that "man's darkest impulses" had returned to Europe and that Americans and their allies must not take the progress of the last 50 years for granted.
"Casual indifference would ignore the lessons that are written in the cemeteries of this continent," Mr. Obama said. "It would allow the old way of doing things to gain a foothold in this young century. And that message would be heard, not just in Europe but in Asia and the Americas, in Africa and the Middle East."