President Obama waves after taking part in a group photo following a meeting with European leaders at the presidential palace in Warsaw on Tuesday. (Reuters)

President Obama pledged his ironclad commitment Tuesday to the defense of Europe and proposed as much as $1 billion in additional spending to bolster the U.S. military presence in Poland and neighboring countries, part of a strategy to reassure nervous allies and check Russia’s encroachment into the region.

Standing beside Polish President Bronislaw Komorowski at the start of a four-day tour of Europe, Obama warned Russian President Vladimir Putin that he will face additional sanctions if he escalates the situation in Ukraine and urged him to take steps to resolve it diplomatically. “We have prepared economic costs on Russia that can escalate if we continue to see Russia actively destabilizing one of its neighbors,” Obama said.

On Friday, Obama and Putin are both set to attend a commemoration of the 70th anniversary of D-Day in Normandy, France. While French President François Hollande and British Prime Minister David Cameron have scheduled meetings with Putin outside the event, Obama has not.

“Mr. Putin has a choice to make . . . that’s what I’ll tell him if I see him publicly,” Obama said.

Even as he proposed new funding to reassure Eastern Europe and put Russia on notice, Obama also sought to send a message to other NATO allies farther to the West who have long resisted increasing their own defense spending.

President Obama landed in Poland on Tuesday at the start of a four-day trip to Europe meant to reassure allies in the wake of the Russian intervention in Ukraine. (Reuters)

As Obama spoke in Warsaw, Defense Secretary Chuck Hagel was lecturing his NATO counterparts in Brussels on “stepping forward . . . when their own security is threatened.”

“As President Obama asks the United States Congress and the American people to support increased investment in European security, we are asking our European allies to do the same,” Hagel said. Only a handful of NATO members meet the alliance benchmark of spending at least 2 percent of their gross domestic product on defense.

The spending argument is not a new one, but administration officials from Obama on down believe they have a potent new argument in Russia’s Ukraine aggression, right at NATO’s doorstep. The alliance, they argue, needs to return to its first principles of defending itself, after two decades of operations far afield in the Balkans and Afghanistan.

Every alliance member “has to do its fair share,” Obama said. As NATO reconsiders its longer-term posture toward Russia, “it’s going to require some flexibility, some additional planning, some joint capabilities that right now we don’t have . . . and it’s going to require every NATO member to step up.” With the exception of Poland and the Baltic countries, “we have seen a decline, steadily, in European defense spending generally,” he said.

Earlier in the day, addressing a joint display of U.S. and Polish troops, Obama made the first of several reassurances about the U.S. commitment to the defense of the new democracies in Eastern Europe. “Our commitment to Poland’s security, as well as the security of our allies in Central and Eastern Europe, is a cornerstone of our own security, and it is sacrosanct,” he said, flanked by F-16s that are part of a joint training program between the two countries.

Eastern European leaders have been at the forefront of alliance arguments that Russia’s annexation of Crimea and its support for separatists in eastern Ukraine have fundamentally changed the security situation in the region and put other countries at risk.

“It is difficult not to notice that something has changed to the east of the borders of NATO, that again they’re having to do with the aggression with the use of armed forces against one’s neighbor,” Komorowski said during the joint news conference. “A few years ago it was Georgia. Now it is Ukraine, with a special focus on Crimea.”

The U.S. money proposed by Obama would cover a continuation, likely into next year, of temporary measures designed to send a signal to Russia and convince Eastern European members that NATO has their back. It includes ongoing land, sea and air deployments, as well as moving additional resources closer to the Russian border and providing additional aid to non-NATO members Ukraine and Georgia.

NATO defense chiefs, meeting in Brussels to draw up plans to be considered at the next alliance summit, scheduled for September in Wales, held lengthy discussions on whether and how to adjust their basic defense plans in light of Russian actions.

“For the last 20 years, we’ve been focused on operations outside of NATO states proper,” a senior U.S. diplomat said at the NATO meeting. As it began to contemplate the end of combat operations in Afghanistan, the alliance has been suddenly “confronted with a reminder” about its own defense that it has not seriously considered since the end of the Cold War, the diplomat said.

While alliance members have contributed to the temporary bolstering of defenses and protection of the newer members in Eastern Europe for which Obama proposed additional U.S. funding, the “conversation we’re only beginning to have today” the diplomat said, is what NATO needs to do over the longer term regarding Russia.

The relationship is now based on a 1997 NATO-Russia partnership agreement in which the West agreed it would not have “substantial and permanent” troops and bases near Russia’s European borders. “It’s quite clear today that we have a different strategic setting” after Russia’s actions in Ukraine, the diplomat said. “Do we still have a partnership? Is the NATO-Russia [agreement] still viable,” or should NATO be adapting to a fundamental change on its eastern flank?

“There’s a lot of space between nothing and ‘substantial and permanent,’ ” the diplomat said.

In a news conference after Tuesday’s meeting at the two-day NATO conference, NATO Secretary General Anders Fogh Rasmussen said all members had helped protect Eastern Europe. He noted that over the long term, Russia’s defense budget had steadily increased while the overall defense spending of NATO countries had decreased by 20 percent. “It’s unsustainable,” Rasmussen said. “What we have witnessed in Ukraine is a wake-up call. And based on that, political leaders, in particular in Europe, should review their defense spending.”

In a speech Wednesday, Obama will mark the 25th anniversary of Poland’s first democratic elections. Some Poles have expressed hope that he will issue a direct challenge to Russia and echo the spirit of past speeches by Presidents Ronald Reagan and John F. Kennedy at the Brandenburg Gate.

Others doubt he will reach those heights, and some have questioned U.S. global leadership. Nobel Peace Prize laureate Lech Walesa, who helped bring democracy to Poland and was its second president, told the Associated Press late last month that “the world is disorganized and the superpower is not taking the lead.”

Obama leaves Poland on Wednesday afternoon to travel to Brussels for a Group of Seven meeting, where Ukraine is likely to remain the dominant subject. He will overnight Thursday in Paris.

On Friday, after bilateral meetings with Cameron and Hollande, Obama is scheduled to attend and make remarks at D-Day commemorations at Omaha Beach in Normandy, paying tribute to U.S. service members who helped liberate France.

DeYoung reported from Brussels. Michael Birnbaum in Moscow contributed to this report.