WASHINGTON (AP) — The U.S. and some of its NATO allies see Russia's intervention in Ukraine as a turning point in the post-Cold War period.

They fear it will destroy two decades of hope that Moscow could be made a lasting partner with the West.

NATO's second-ranking official said Thursday that the alliance is now compelled to view Russia as an adversary.

Alexander Vershbow, the American deputy secretary general of NATO, told reporters that Russia's annexation of Crimea and its apparent efforts to further destabilize eastern Ukraine pose what he calls "grave challenges" to the international security system that has developed since the end of World War II.

Defense Secretary Chuck Hagel is delivering a speech on the Ukraine-Russia situation Friday at a Washington think tank.