Pablo Martinez Monsivais/AP

President Obama may be considering sending weapons to Ukraine to help its soldiers repel Russian-backed separatists, but he isn't drawing a "red line" on what would prompt him to do so. The subject of a red line in foreign policy is, of course, an uncomfortable one for the president, who infamously ignored the bright demarcation he set when the Syrian army was caught using chemical weapons against rebels in 2013.

During a press conference Monday with German Chancellor Angela Merkel, Obama made clear that sending defensive arms to Ukraine was one of the options his team was mulling if neither diplomacy nor economic sanctions persuades Vladimir Putin to leave Ukraine alone. That would undoubtedly represent a significant escalation of U.S. involvement in the conflict, and when asked by a German reporter about a "red line," Obama said no. "There's not going to be any specific point at which I say, 'ah, you know, clearly lethal defensive weapons would be appropriate here,'" the president replied. "It is our ongoing analysis of what can we do to dissuade Russia from encroaching further and further on Ukrainian territory."

The exchange was notable because of the presence of Merkel, who during a speech on Saturday explicitly rejected the possibility of sending arms to Ukraine on the grounds that it would make the situation worse, and that winning a military conflict with Russia was impossible. She stuck to that position Monday at the White House, even as Obama refused to rule it out. (Merkel said she wanted to make at least one more attempt to broker a ceasefire in the region.) The president is also facing increasing pressure domestically to bolster assistance to Ukraine, and his nominee for defense secretary, Ashton Carter, told a Senate committee last week that he was "very much inclined" toward arming Ukraine.

The debate over arming Ukraine underscores just how frustrated Obama and U.S. allies in Europe are over their inability to force Putin's hand, even in the face of sanctions that have helped tank the Russian economy. Instead, the president and the chancellor were left to emphasize their unity in confronting Putin, despite the tactical difference on the arms issue. "We can't simply try to talk them out of it," Obama said at one point. "We have to show them that the world is unified in imposing a cost for this aggression. And that's what we're going to continue to do."

At the same time, there was little disagreement between Obama and Merkel on the question of whether arming Ukraine would actually allow them to defeat Russia in a military conflict. The president conceded that was "unlikely" given that the Russian military remains "extraordinarily powerful." The point of sending arms would be to ratchet up the costs of the Russian intervention in eastern Ukraine, persuading Putin to withdraw or at least strike a diplomatic agreement. Obama didn't sound all that confident that dispatching weapons would work, which was all the more reason to keep the red pen in his pocket.

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