The Russian leader in recent months has been outlining what he describes as historical grievances and betrayals by Western nations in the post-Soviet period that Moscow will no longer tolerate. Actions by NATO and the United States, he has suggested, have left Russia little choice but to threaten to use the military. Russia is obliged, he has said, to protect Russians left stranded outside the country by the breakup of the Soviet Union three decades ago.
Understand the Escalating Tensions Over Ukraine
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Ominous warnings. Russia called the strike a destabilizing act that violated the cease-fire agreement, raising fears of a new intervention in Ukraine that could draw the United States and Europe into a new phase of the conflict.
The Kremlin’s position. President Vladimir V. Putin of Russia, who has increasingly portrayed NATO’s eastward expansion as an existential threat to his country, said that Moscow’s military buildup was a response to Ukraine’s deepening partnership with the alliance.
On Tuesday, in comments that appeared to suggest an imminent threat, Russia’s defense minister, Sergey K. Shoigu, said that American military contractors had appeared in eastern Ukraine, near two Russia-backed separatist regions, and that they had brought with them “an unknown chemical component.”
But possibilities for an escalation abound. Mr. Putin, speaking at the same event with Mr. Shoigu on Tuesday, ruminated on the possibility that the United States had long-term plans to deploy hypersonic missiles in Ukraine, something that the United States has never suggested it intends to do.
“What they are now doing on the territory of Ukraine, or trying to do, or planning to do, is not thousands of kilometers from our national borders,” Mr. Putin said. “It’s on the doorstep of our home. They just have to understand that we have nowhere left to retreat.”
Anton Troianovski reported from Moscow, and Andrew E. Kramer from Kyiv, Ukraine.