President Barack Obama is welcomed by Belgian Prime Minister Elio Di Rupo, right, and King Philippe, left, after his arrival at Brussels Airport, in Brussels, Belgium, March 25, 2014. (Julien Warnand/EPA)

Barack Obama has spoken to the continent of Europe on several occasions — as a hopeful presidential candidate in 2008, as a new president memorializing the dead at Normandy, and most recently as a second-term president hoping to stir his allies from a complacency that he warned hovers over the prosperous developed world.

He will do so again Wednesday evening at a time when, far from complacent, Europe is fearful. Russian President Vladimir Putin has revived Cold War memories by pushing into Crimea and massing troops along the border with eastern Ukraine. As he has during his first two days in Europe, Obama will argue here for a response, from a fractious European Union, equal to Putin’s challenge.

Obama will meet with European Union and NATO officials in his first visit to the bureaucratic heart of post-Cold War Europe, a vision of borderless trade, common currency, and a mobile labor pool that he said this week at The Hague is threatened by Putin’s military turn.

But he will make the case most publicly — for unity, for sanctions that would damage the continent’s still-fragile economy, for help leveraging American power that in this case does not include military force — in a speech Wednesday evening at the Palais des Beaux-Arts. The museum will serve as a cultural counterpoint to Putin’s display of force.

A senior administration official, speaking on condition of anonymity to preview the president’s remarks, said “the speech itself is an opportunity for him to step back and look at the current events in Ukraine in a broader context.”

“Standing at the heart of Europe, in Brussels, the center of the European project, he’ll be able to speak about the importance of European security, the importance of not just the danger to the people of Ukraine but the danger to the international system that Europe and the United States have invested so much in, as a consequence of Russia’s actions,” the official said. “He will be able to speak more broadly about why the alliance between Europe and the United States is so important to Europe’s security but also the progress of democracy and the sustainment of international law all around the world.”

Over the course of his presidency, Obama has looked to the big stage, particularly the international stage, to set out challenges (Prague in 2009 where he outlined his aspiration to rid the world of nuclear weapons), change the tone of U.S. diplomacy (Cairo two months later when he asked the Muslim world for a “new beginning”), or explain himself (Oslo late that year when he argued in support of “just war” in accepting the Nobel Prize for Peace.)

Less than a year ago, Obama spoke before the Brandenburg Gate, the first U.S. president to do so from what had been East Berlin. He celebrated the wall’s collapse, along with Cold War geopolitics, with a warning to the audience gathered under a brutal summer sun to see him.

“And yet, more than two decades after that triumph, we must acknowledge that there can, at times, be a complacency among our Western democracies,” Obama said. “Today, people often come together in places like this to remember history — not to make it.”

The speech was largely a celebration of progress. But Obama also spoke about how that same progress had turned Europe, and the United States, away from the disparities in income, in freedom and in national trajectories that have grown in the years since the wall came down.

“We face no concrete walls, no barbed wire. There are no tanks poised across a border,” he said. “And so sometimes there can be a sense that the great challenges have somehow passed.  And that brings with it a temptation to turn inward — to think of our own pursuits, and not the sweep of history — to believe that we’ve settled history’s accounts, that we can simply enjoy the fruits won by our forebears.”

Putin’s move into Crimea suggests a historic account unsettled, and a military method of settling it that Obama has called out of place in this century. His warnings that day resonate as he prepares, again, to describe the importance of the American-European alliance at a more urgent moment on the continent.

  “Our values call upon us to care about the lives of people we will never meet,” he said in Berlin. “When Europe and America lead with our hopes instead of our fears, we do things that no other nations can do, no other nations will do.”

Obama began Wednesday moving back even further into Europe’s 20th century — visiting Flanders Field to lay a wreath at the memorial for the 368 Americans killed on among the grimmest of World War I battlegrounds.

He spoke, briefly, alongside Belgian Prime Minister Elio Di Rupo and King Philippe, who guided him through the cemetery and fields, now sown with red poppies above the bodies of tens of thousands of fallen soldiers.

“It is impossible not to be awed by the profound sacrifice they made so that we may stand here today,” Obama said, adding that “here we saw that no solider — and no nation — fought alone.”

At the end of the day, Obama will prompt Europe to look from Flanders Field to its far eastern edge, asking what the allies must do this time to stop a broader war.