"Russia's actions in Ukraine shatter that myth and usher in bracing new realities," Hagel said in a speech that captured the Obama administration's deepening concern that decades of effort to draw Russia closer to the West may be failing.
Hagel made no mention of boosting American troop levels in Europe; his focus was on how NATO can become better prepared to deal with a security landscape that is being reshaped by Russia's annexation of Crimea and its moves in eastern Ukraine.
Tensions in Ukraine escalated further. Two Ukrainian helicopters were shot down as the interim government in Kiev launched its first major offensive against an insurgency that has seized government buildings across eastern Ukraine. The Kremlin said Kiev's military move against the insurgents "destroyed" the two-week-old Geneva agreement on cooling Ukraine's crisis.
In a speech at the Woodrow Wilson Center for International Scholars, Hagel expressed satisfaction with the way European allies have responded to Russian actions in Ukraine.
"But over the long term," he said, "we should expect Russia to test our alliance's purpose, stamina and commitment." He called this a "clarifying moment" for a NATO alliance that had grown used to the idea of a benign Russia to its east.
In remarks at the White House, visiting Germany Chancellor Angela Merkel said the "post-Cold War order has been put in question" by Russia's aggressive moves.
As Europe worries that Russia may be preparing for broader aggression, Hagel said NATO "must stand ready to revisit the basic principles underlying its relationship with Russia." He did not elaborate. NATO has had a formal relationship with Russia since 1997, although last month the alliance declared that it was suspending cooperation with Moscow in light of the Ukraine problem.
Asked later whether Hagel believes Russia is an enemy of the U.S., his press secretary, Navy Rear Adm. John Kirby, said the nature of the relationship is up to Russia, since its actions are chiefly responsible for the destabilizing of eastern Ukraine.
"There is no reason for us to consider Russia an enemy unless Russia wants to declare itself one, and I can tell you the path they are taking in Ukraine is certainly not going in the right direction," Kirby said.
On Thursday, NATO's second-ranking official, Alexander Vershbow, an American, said Russia has compelled the alliance to begin viewing it as an adversary.
Kirby said that inside the Obama administration there is no serious discussion of permanently increasing U.S. troop levels in Europe. The U.S. has sent small numbers of combat troops to participate in exercises in Poland and other NATO countries, and additional measures aimed at reassuring allies are under consideration.
Hagel renewed a familiar U.S. call for increased defense spending by European members of NATO, saying that even a united Europe still faces great dangers.
"While we must continue to build a more peaceful and prosperous global order, there is no postmodern refuge immune to the threat of military force," Hagel said. "And we cannot take for granted — even in Europe — that peace is underwritten by the credible deterrent of military power."
Hagel said European NATO members need to bear a greater share of the collective defense. He said U.S. defense spending is three times the combined spending of its allies.
"In recent years, one of the biggest obstacles to alliance investment has been a sense that the end of the Cold War ushered in the 'end of history' and an end to insecurity — at least in Europe — from aggression by nation-states. Russia's actions in Ukraine shatter that myth and usher in "bracing new realities," Hagel said.
He dismissed the notion advanced by some critics that U.S. support for the expansion of NATO, starting in the late 1990s, is responsible for Russia's recent aggressive moves. Poland and other central and eastern European nations that once were in the Soviet sphere of influence are now NATO members.
"The historical record now speaks clearly for itself," he said. "And it makes clear that NATO has sought partnership, not conflict, with Russia." He said the U.S. and its allies made a "good-faith effort to convince Russia that our security interests were converging," even as Washington worried that Moscow might one day abandon democracy.
Looking beyond the military aspects of European security, Hagel said the U.S. and Europe should work together to "blunt Russia's coercive energy policies." He said the U.S. Energy Department has conditionally approved export permits for U.S. liquefied natural gas that add up to more than half of Europe's gas imports from Russia.
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