Some American officials assert that as a matter of international law, the provision of weaponry and intelligence to the Ukrainian Army has made the United States a cobelligerent, an argument that some legal experts dispute. But while Mr. Putin has made threats about launching attacks to impede the military assistance, he has not yet acted to stop it by attacking bases in neighboring countries — NATO allies — where the equipment originates.
That could change, U.S. officials said, especially if Mr. Putin thinks he is cornered or in danger of losing.
“It is a fine line the administration is still walking in every dimension of its support for Ukraine,” said Andrea Kendall-Taylor, a former senior intelligence official who specialized in Russia and is now at the Center for a New American Security. “They are trying to figure out how do you get right up to the line without crossing over in a way that would risk direct confrontation with Russia.”
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