US reveals Europe military plan
The announcement came as tensions in the region simmer over Russia's aggressive actions in Ukraine.
Standing with Polish president Bronislaw Komorowski, Mr Obama said the US plans to send more military equipment and rotate additional American troops into the region. He called on lawmakers back in Washington DC to provide the funding to sustain the effort.
"Today, I'm announcing a new initiative to bolster the support of our Nato allies here in Europe," Mr Obama said at Warsaw's Belweder Palace. "Under this effort, and with the support of Congress, the United States will preposition more equipment in Europe."
If approved, the funding will be used to increase military exercises and training missions on the continent, as well as rotations of air and ground forces, the White House said.
Officials said Mr Obama was also seeking to ramp up US Navy participation in Nato deployments in the Black and Baltic Seas, plus working to boost the military capacity of non-Nato countries that sit on Russia's border, including Ukraine, Georgia and Moldova.
Mr Obama's announcement came at the start of a three-country swing through Europe steeped in both historical significance and regional anxiety over the crisis in Ukraine.
A day before his first face-to-face encounter with Ukraine's newly elected president, Petro Poroshenko, Mr Obama said he wants both the US and Ukraine to have good relations with Russia. But in a warning to Moscow, Mr Obama said the US has contingency plans to protect every member of Nato, and has been steadily developing those plans in recent years.
"Our contingency plans are not just pieces of paper on a shelf," Mr Obama said, adding that the US must and does have the ability to put those plans into effect if needed.
At the same time, he called on other Nato members to step up by increasing their own role in the alliance's collective defense, even as he acknowledged that the US has greater capabilities to bear that burden than its smaller allies.
"Everyone has the capacity to do their fair share, to do a proportional amount to make sure we have the resources, the planning, the integration, the training in order to be effective," Mr Obama said.
To that end, Mr Komorowski announced that Poland intends to increase its own defence budget, up to 2% of the nation's gross domestic product. "It means it's a very tangible, very clear engagement," Mr Komorowski said.
The cautionary notes from Mr Obama and Mr Komorowski came just a few days before a potential encounter between Mr Obama and Russian president Vladimir Putin, who also planned to be in France for the 70th anniversary of the D-Day invasion that eventually led to Allied victory in the Second World War.
The pair have not met in person since the crisis began and have no meetings together scheduled, but White House officials haven't ruled out that they could cross paths.
Calling his relationship with his Russian counterpart "businesslike", Mr Obama said it is possible for the US to rebuilt trust with Mr Putin, but that doing so would take time and require Mr Putin to use his influence to calm unrest in eastern Ukraine.
"We are interested in good relations with Russia. We are not interested in threatening Russia," Mr Obama said.
But he echoed previous warnings from the US and other Western nations that "further provocation will be met with further costs". The US and Europe have already levied sanctions against Russian officials, but are holding off on further sanctions amid Mr Putin's vow to respect the results of Ukraine's recent presidential election.
Nato secretary general Anders Fogh Rasmussen hailed Mr Obama's announcement of a bolstered US presence on the continent.
"The United States has reacted swiftly after Russia's illegal military actions in Ukraine," Mr Rasmussen said as he met with Nato defence ministers in Brussels. "And I appreciate that other allies have followed so that we can announce that all 28 allies are now contributing to reassurance measures."
Mr Obama's visit to Warsaw coincides with the 25th anniversary of Poland emerging from communism. He also plans to meet Group of 7 leaders in Brussels before heading to France to mark the D-Day anniversary.
Later today, Mr Obama and Mr Komorowski plan to hold discussions on central European security with leaders from Bulgaria, the Czech Republic, Estonia, Croatia, Hungary, Latvia, Lithuania, Romania and Slovenia.
US secretary of state John Kerry has joined Mr Obama for many of his events in Warsaw. During a separate meeting with Poland's foreign minister, Mr Kerry said the crisis in Ukraine presents "a new moment of challenge for all of us".
"Events in Ukraine have unfortunately unleashed forces that we had all hoped had been put away, were behind us, and so it requires new vigilance and it requires clear commitment," he said.