President Biden will warn Russian President Vladimir Putin when they speak on Tuesday that if Russia invades Ukraine, the U.S. is prepared to increase its troop presence, capabilities and military exercises on NATO's "eastern flank," a senior administration official told reporters.
Why it matters: The administration believes it's increasingly likely that Putin will order an invasion.
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Biden is not threatening to respond militarily to defend Ukraine, but to hit Putin with sanctions, bolster the defenses of NATO allies like the Baltic states, and send Ukraine more equipment to defend itself.
The big picture: Russia has deployed tens of thousands of troops at various points near the border, and is "stepping up its planning for significant military action against Ukraine," the senior official said on a briefing call to preview the Biden-Putin conversation.
The White House has also warned that the Kremlin is stepping up a disinformation campaign against Ukraine, potentially as a pretext to blame Kyiv for any military clash.
"We do not know whether President Putin has made a decision about further military escalation in Ukraine. But we do know that he is putting in place the capacity to engage in such escalation should he decide to do so," the official said.
The other side: Putin has set out red lines of his own ahead of the call, including the potential deployment by NATO of offensive missiles systems on Ukrainian territory.
He's also seeking a legally binding guarantee that NATO will not expand eastward, including to Ukraine.
In addition, Moscow has objected to NATO exercises near the Russian borders, and increased cooperation between the alliance and Ukraine.
Between the lines: Ukraine is seeking membership in the alliance and Putin is seeking guarantees that Ukraine will never be invited to join. Both prospects currently appear remote.
Whether or not Putin orders an invasion, his troop movements have moved Russia from a secondary priority to the top of Biden's in-tray.
Asked if the U.S. military could intervene directly if Russia invades, the senior official said that the U.S. was "not seeking to end up in a circumstance in which the focus of our countermeasures is the direct use of military force."
The official reiterated that the focus would instead be sanctions, coordinated with Europe, as well as support for the Ukrainian military and "a substantial increase in support and capability to our NATO allies to ensure they remain safe."
The bottom line: Putin has proved time and again that he is not deterred by Western sanctions, but he may be deterred by the high military cost of invading Ukraine, says Dmitri Trenin of Carnegie Moscow.