Standing side by side with a pair of Baltic leaders in Vilnius, Lithuania, Biden said the U.S. was "absolutely committed" to defending its allies, adding that President Barack Obama plans to seek concrete commitments from NATO members to ensure the alliance can safeguard its collective security.
In a jab at Russia, he said the U.S. stands resolutely with Baltic states in support of the Ukrainian people against Russian aggression.
"Russia cannot escape the fact that the world is changing and rejecting outright their behavior," Biden said, after meeting in Vilnius with Lithuanian President Dalia Grybauskaite and Latvian President Andris Berzins.
Biden's comments came at the close of a two-day trip to Lithuania and Poland with a two-pronged theme: Sending a stern message to Russian President Vladimir Putin that the U.S. won't abide Russian intervention in Ukraine, and reassuring fretful NATO allies that the U.S. and others will come to their defense if necessary.
"We're in this with you, together," Biden said.
Amid the tough talk from Biden and the Baltic leaders, Russia's annexation of Crimea was increasingly looking like a foregone conclusion.
At the Ukrainian navy headquarters in Sevastopol, Crimea, militias stormed the base Wednesday, taking it over without resistance. Although senior Ukrainian officials planned to travel to Crimea in hopes of averting an escalation in hostilities, Crimea's pro-Russian prime minister insisted they weren't welcome and wouldn't be allowed to enter.
A day earlier, Putin declared Crimea part of Russia in a passionate speech steeped in Russia's sense of being slighted and marginalized by the West in the years since the Cold War.
While repeatedly insisting that Russia's move is illegal and won't be recognized, the U.S. and other world powers have also turned their attention to eastern Ukraine and other areas with large ethnic Russian populations, lest Putin seek additional territory in what some fear could portend a return to Moscow's traditional imperialist ambitions.
To that end, Western powers were seeking fresh ways to show that Russia would incur real costs unless it changes course.
Berzins announced that he and Polish President Bronislaw Komorowski had agreed that Poland and Latvia will start coordinating its security activities more closely. France's foreign minister said leaders of the Group of Eight world powers have suspended Russia's affiliation with the group over its actions in Ukraine. Obama invited the seven other members to discuss what comes next during an emergency meeting in Europe.
Meanwhile, Britain said it was suspending military cooperation with Russia in light of the crisis. And German Chancellor Angela Merkel, in a phone call with Obama on Tuesday, agreed that U.N. and other international monitors must be sent in to other parts of Ukraine without delay.
At the same time, the U.S. and its partners were seeking to mount a more visible show of NATO's military might in the region, despite Putin's insistence that he has no intention of invading other regions in Ukraine, much less other nations.
Biden announced in Warsaw that in addition to new NATO exercises that will take place in Poland, the U.S. was considering rotating American forces to the Baltic region as a step toward ensuring the collective defense of NATO allies. Those forces could conduct ground and naval exercises, and engage in training missions. At Warsaw's request, the U.S. last week sent some 300 air troops and a dozen F-16 fighters to Poland for joint training.
"This situation is a direct threat to our regional security," Grybauskaite said, denouncing "the use of brutal force to redraw the map of Europe."
NATO's modern role in global security has come into renewed focus with the eruption of tensions in Ukraine, as NATO members like Poland and the Baltics question how deeply they can rely on an alliance that has more recently been focused on other entanglements, like the war in Afghanistan. Almost 10 years to the day after Lithuania and Latvia joined NATO, the Baltics are suddenly plunged into the type of eerie concern about foreign aggression they may have thought they'd left behind at the end of the Cold War.
Firm rhetoric, sanctions and travel bans have not been enough so far to dissuade Putin and his military from seizing control of Crimea and then, after a Crimean referendum that the West condemned as illegal, declaring it part of Russia. Other countries have been watching warily out of concern they could be next.
"The punishment doesn't fit the crime, and the Baltic states and central European states know this," said Michael Geary, a European relations analyst at the Wilson Center, a Washington think tank. "They're worried that the U.S. response has been mediocre at best, and there's a palpable sense they need reassurance. Will they be protected in the event of further westward march by Russia?"