Joshua Keating gets  a quote from Barry Posen on the folly of arming Ukraine:
If Russia is willing to bear the costs of political and economic isolation, which for now it seems to be, it can continue to pour resources into the fight, knowing that there’s only so far Western countries will be willing to go to counter them. “I think there’s a real risk,” Posen says, “that we’re going to start providing weapons to the Ukrainians and will look up after a year or so and find out it hasn’t really helped that much.”
There is also the matter of getting the weapons to Ukrainian forces and training them in their use in a timely fashion. As Micah Zenko noted  in a recent post, to do this before this spring “would be a highly ambitious, if not unprecedented, political and logistical effort.” Considering how slowly U.S. arms have reached anti-regime Syrians once the administration decided to start providing them with weapons, it is very doubtful that the weapons would reach Ukraine quickly enough to even try to do what the hawks want.
Like Posen, I don’t expect that sending arms to Ukraine will have the desired effect. Even if it doesn’t lead to escalation and a severe worsening of the conflict, which is very well might, it still isn’t going to “work” in the way that its advocates imagine. When it doesn’t work as intended, we know that this won’t discourage the hawks in the least, but will prompt them to demand that the U.S. “do more.” If they lure the administration into making the first blunder of agreeing to send some weapons, they will then insist that the U.S. cannot now “abandon” the people it has been arming.
The debate over arming Ukraine has a lot in common with the debate over arming rebels in Syria. Hawks insist from the start that the U.S. needs to be arming one side in a foreign conflict, whose importance to the U.S. is grossly exaggerated to make it seem imperative that the U.S. does what they want, and they dismiss any possible negative consequences while focusing solely on the supposed benefits of “action.” Once the warnings of skeptics of U.S. involvement are proven correct, that doesn’t weaken the hawks’ desire to throw more weapons at the problem, but just makes them even more certain that the U.S. has to keep increasing its support for its ineffective proxies.