THE ARMS DEAL THAT ALMOST DIDN’T HAPPEN…. The Obama administration was delighted to announce the framework of a new nuclear arms treaty with Russia yesterday, but Peter Baker has a fascinating account detailing how the deal almost didn’t happen.

President Obama was angry. He was on the phone with President Dmitri A. Medvedev last month to finalize a new arms control treaty with Russia, only to be confronted with new demands for concessions on missile defense. A deal that was supposed to be done was unraveling.

“Dmitri, we agreed,” Mr. Obama told Mr. Medvedev with a tone of exasperation, according to advisers. “We can’t do this. If it means we’re going to walk away from this treaty and not get it done, so be it. But we’re not going to go down this path.”

Mr. Obama hung up and vented frustration. Some of his advisers had never seen him so mad. A picture taken by a White House photographer captured his grim face in that moment of uncertainty. For a year he had been trying to forge a new relationship with Russia, starting with a treaty to slash nuclear arsenals. And for a year Russia had been testing him, suspecting he was weak and certain it could roll over him.

If Mr. Obama overestimated his powers of persuasion in reaching quick agreement with the Russians, they misjudged how far they could get him to bend.

That seems to be one of the key takeaways from the long-sought treaty — Russia wanted to test Obama, and hoped he’d buckle. Indeed, the Russians seemed to think it was likely, given the U.S. president’s domestic challenges, and Obama’s desire to complete the deal and move on.

“When President Obama’s domestic positions were weakened in recent months and he was completely consumed in his crusade for health care reform, making all other issues irrelevant, it is surprising how much attention he kept on Start,” said Sergei M. Rogov, director of the Institute for U.S. and Canada Studies in Moscow, referring to the treaty. “Even being 24-hours-a-day busy on health reform, he had a 25th hour for Start.”

What Russia didn’t expect is what they got — a U.S. president who took hard lines, didn’t yield, and was willing to be patient. That was true even when missile defense provisions threatened to scuttle the entire deal.

Dmitri V. Trenin, director of the Carnegie Moscow Center, said that the Kremlin thought Mr. Obama would back down out of eagerness to finish the treaty before coming international nuclear summit meetings.

“They believed Obama could be put under pressure and concessions could be extracted from him,” Mr. Trenin said. “He needed the treaty more than the Russians in the short term.”

“If you’re going to continue to persist on this missile defense language, we’re going to have to walk away,” Obama said to his Russian counterpart, according to senior officials.

The Russians kept up the hard-line posture in private as well, and after President Obama threatened to walk away from the negotiating table if Russians continued to insist on including missile defense in the treat, White House officials say they didn’t know what would happen.

Eventually Russians dropped the issue.

“At the end of the day it was a pivotal moment,” the official said, suggesting that the Russians saw President Obama as someone who wasn’t going to “cave.”

Despite the Russians’ apparent leverage, Obama didn’t back down. The Russians did.

Periodically over the last few years, Obama has joked that “just because I’m skinny doesn’t mean I’m not tough.” I think everyone is starting to get a better sense that the boast is accurate.

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.