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 its neighbors, 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 crisis 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,” said Obama, who will see the Russian leader Friday at a summit marking the 70th anniversary of D-Day. “Mr. Putin has a choice to make.”

Earlier, addressing a joint display of U.S. and Polish troops, Obama gave the first of several reassurances about 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.

U.S. President Barack Obama is greeted by Poland's President Bronislaw Komorowski as he arrives in Warsaw, Poland, Tuesday, June 3, 2014. (Charles Dharapak/AP)

European leaders, especially in the east, have expressed anxiety that the United States will not want to focus attention or resources on the Russian threat — a concern that intensified after Obama seemed to narrow the case for using military action in a high-profile speech at the U.S. Military Academy at West Point last week.

His trip this week, which includes meetings with a host of Eastern and Central European leaders, Ukrainian President-elect Petro Poroshenko and Group of Seven allies, is geared toward making clear that he is in no way abandoning longtime U.S. commitments to Europe.

His proposal to spend as much as $1 billion on increased military exercises in Eastern Europe, which must be authorized by Congress, was well received by Eastern European leaders, although it fell short of their wish that the United States would establish a permanent military presence in Eastern Europe.

European leaders argue that Russia’s annexation of Crimea in March and support since then for pro-Russian separatists in eastern Ukraine have dramatically changed the security situation in Eastern Europe 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. A few years ago, it was Georgia; now, it is Ukraine, with a special focus on Crimea,” Komorowski said in a joint news conference with Obama. “President Putin didn’t hide it — he didn’t hide that these were elements of the Russian armed forces, and this is something that we have to acknowledge, just the same way Russia never hid it that for the last four years, it has increased its defense budget two-fold.”

In his remarks, Obama said he expects Russia to undertake several actions to de-escalate the situation in eastern Ukraine, beyond moving its troops away from the border.

“Further Russian provocation will be met with further costs for Russia, including, if necessary, additional sanctions. Russia has a responsibility to engage constructively with the Ukrainian government in Kiev to prevent the flow of militants and weapons into eastern Ukraine,” Obama said. “Russia also needs to be using its influence with armed separatists to convince them to stop attacking Ukrainian security forces, leave buildings that they’ve seized, lay down their arms and enter into the political process.”

The U.S. proposal, to be included in an upcoming Pentagon spending request, would fund what the White House calls a “European Reassurance Initiative.” It would be used to increase military personnel deployed to Eastern Europe, who would in turn assist with training and strategic planning.

In addition, the White House is also seeking to reposition equipment and infrastructure within Europe and increase participation by the U.S. Navy in NATO deployments in the Black and Baltic seas. Finally, the Obama administration is looking to build the military strength of Georgia, Moldova and Ukraine so they can better provide for their own defense.

The United States has taken other modest steps to increase its presence in the region, including rotating 600 paratroopers through Estonia, Latvia, Lithuania and Poland for joint training exercises, beginning in April.

Obama will speak in Poland on Wednesday to mark the 25th anniversary of the nation’s first democratic elections. Some Poles have expressed hope that his speech will be in the spirit of presidents Ronald Reagan and John F. Kennedy at the Brandenburg Gate in Berlin decades ago, though others doubt that he will reach for those rhetorical heights.

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 G-7 meeting, where Ukraine is likely to remain the dominant subject.

Obama is likely to continue to put pressure on his European counterparts to be prepared to intensify sanctions against Russia if it escalates its intervention in Ukraine, although no new sanctions are planned in the near future.

At the same time, Europeans will pressure Obama to take steps to help reduce reliance on Russia for energy by urging him to permit the export of liquefied natural gas from the United States, a move that is opposed by many environmental organizations.

After a bilateral meeting with British Prime Minister David Cameron on Thursday, Obama travels to Paris for dinner with French President François Hollande.

On Friday, Obama is scheduled to attend and make remarks at D-Day commemorations at Omaha Beach in Normandy — a visit that will pay tribute to U.S. service members who helped liberate France. The visit, however, comes in the shadow of a scandal over wait times and doctored records at the Department of Veterans Affairs in the United States.