President Obama warned European leaders Wednesday that nations must “chip in” fairly to ensure a NATO capable of deterring an expansionist Russia, and he placed the responsibility largely on the continent to resolve its dependence on Russian energy.

Speaking at a news conference after meeting with European Union leaders, Obama noted that he has been concerned by declining defense budgets among some NATO members, a complaint he has allowed other administration officials to make in the past.

His words were a pointed reminder that despite U.S. involvement in seeking to prevent Russian President Vladimir Putin from advancing further beyond Russia’s borders, European leaders must be ready to pay more for their defense.

“If we have collective defense, it means everyone’s got to chip in,” Obama said, appearing after meeting with Herman Van Rompuy, president of the European Council, and Jose Manuel Barroso, president of the European Commission.

Obama cited declining “trend lines” in defense spending among some NATO members, cutbacks he called expected given the financial straits that many European nations have found themselves in over the past five years.

But he said members of NATO must now recommit to defense spending, especially as the United States enters the final months of its post-Sept. 11, 2001, wars. Obama is heading to a NATO meeting later Wednesday.

“Our freedom isn’t free, and we have to be able to pay for the assets, the personnel, and the training to make sure we have a credible NATO force and an effective deterrent force,” Obama said. “Everyone is going to have to make sure they are engaged and involved, and I think that will help build more confidence among member states.”

As Obama began his first visit here, European and Ukrainian officials pushed ahead on Wednesday with a plan to strengthen ties, a task made more urgent by Russia’s recent takeover of Crimea and concern over further threats.

European Union officials were in Kiev on Wednesday as a standoff with Russia continued over its absorption of Crimea, and amid efforts by the United States and its allies to isolate Russia for its actions.

Ukraine signed an Association Agreement with the E.U. last year — in the process spurning a closer economic alliance with Russia. A last-minute refusal to sign the agreement in November by former President Viktor Yanukovych led to weeks of protests and his overthrow in February — triggering Russia’s move into the Crimean Peninsula.

The detailed work on a political association between Ukraine and the E.U. is running parallel to economic reform negotiations with the International Monetary Fund to pave the way for a loan package of as much as $15 billion.

In tandem, the two sets of negotiations are meant to insulate Ukraine’s new government against economic pressure from Moscow, and to draw the country closer to Western Europe. The political agreement is to include measures that will bring Ukrainian courts and other institutions in line with European standards. Free trade and other aspects of the agreement are to be discussed after a May 25 presidential election.

Tensions between Ukraine and Russia remained high Wednesday, with Russia accusing Ukraine of forcing crews of Aeroflot flights to Ukraine to remain on board their planes instead of disembarking as usual.

This is Obama’s 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.

He 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.

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.

Among those measures will be a new seriousness among European leaders to diversify their energy imports, now heavily reliant on Russian oil and natural gas.

On Wednesday, Barroso called Russia’s move on Crimea “a real wake-up call” for European leaders on the issue of energy. Although Obama pledged in the meeting Wednesday to help Europe think through energy strategies — and held out the potential of expanding U.S. natural gas exports to Europe — he made clear that the bulk of the burden must be managed by Europeans.

“Here in Europe we must do our homework,” Barroso acknowledged.

Obama 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 Peace Prize).

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 one of the grimmest 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.

Lally reported from Kiev.