(JEWEL SAMAD/AFP/Getty Images)

In his address to the United Nations on Tuesday, President Obama did his best to rally the organization to action on Syria. His case was forceful but, at moments, the logic seemed strained, even contradictory. And it was all made a bit awkward by the fact that Obama's urgent call to action came more than two years into the war, after two far milder U.N. addresses.

There were two contradictions in Obama's comments to the United Nations on Syria. The first was with the Obama of General Assemblies past, who espoused a very different view of the war and how to handle it. Previously, Obama had not advocated any of the military and diplomatic actions that, today, he declared so vital that failing to pursue them could undermine the legitimacy of the United Nations itself. The second contradiction was in Obama's two goals in Syria – punishing Assad for his chemical weapons and ending the war – which he framed as complimentary even though they would appear to work at cross-purposes.

This gets to the bigger, underlying contradiction: Obama has a habit of conflating his case for punishing chemical weapons use with his case for ending the war, and says we can do both at the same time. But he advocates contradictory actions in pursuit of those two goals.

To be clear, this is not to argue that Obama is hypocritical or somehow dishonest. But he's got a very tough needle to thread: he's trying to rally an action-resistant United Nations into very difficult and unpopular action; he's also trying to push it toward two very different forms of action. Those are really difficult goals. That Obama is back-bending through some less-than-consistent rhetoric is a sign of just how difficult.

Still, the shift in Obama's position is revealing. Just one year ago, in his United Nations General Assembly speech, Obama said of Syria only that "we must stand with those Syrians who believe in a different vision." The war, at that point, was already horrifically violent; President Bashar al-Assad's forces had not used chemical weapons but they had committed plenty of the slaughter that Obama cited today as cause for action. Yet, in his previous addresses, he'd made no call for action, no declaration that the "legitimacy" of the U.N. was on the line, as he argued today.

If the United Nations Security Council failed to pass a sufficiently tough resolution to force Assad to give up his chemical weapons, Obama warned, "then it will show that the United Nations is incapable of enforcing the most basic of international laws." Those are pretty high stakes, after two years of relative U.S. inaction on Syria, despite tens of thousands killed. Obama's prior U.N. addresses since the war began, in 2011 and 2012, somewhat undermined his big call to action today. In those two speeches, he did not demand U.N. action – nor pledge any concrete U.S. steps.

You could argue that Assad's alleged use of chemical weapons on Aug. 21 changed all that, justifying Obama's radically different approach. But Obama, in making his case for action today, cited not just chemical weapons growing sectarianism, the danger of regional destabilization, extremism and the larger human costs of the war. Those were all present a year ago. And Obama argued for specific action not just to end chemical weapons but to end the war itself – which did not seem to merit the same sort of response for him last year.

On paper, Obama's two overriding goals in Syria are actually pretty straightforward. First, he wants to uphold the international norm against the use of chemical weapons, which he believes Assad violated by using chemical weapons against civilians on Aug 21. Second, he wants for Assad to step down voluntarily as part of a negotiated peace deal with the rebels that would also leave elements of Assad's government intact.

The problem comes when Obama explains how to achieve those goals. He told the United Nations today that the threat of force could compel Assad to give up his chemical weapons, but that actual military force could not end the war. He argued, on the one hand, "I do not believe that military action by those within Syria or by external powers can achieve a lasting peace." On the other, he said that only the threat of military strikes had compelled Assad to accept the chemical weapons deal.

One might reasonably conclude, taking Obama's arguments at face value, that the world would have to pursue these goals separately. At one point, it would have to pick: threaten and maybe use force to get rid of the chemical weapons, or instead of force pursue a diplomatic peace deal.

The problem, though, is that Obama has linked his two pursuit, saying that the one complements the other. "Our agreement on chemical weapons should energize a larger diplomatic effort to reach a political settlement within Syria," he said. That's a bit of a contradiction: military force would undermine a peace deal, but it would force Assad to give up his chemical weapons and thus "energize" a peace deal.

How do you square that circle? Some hawkish analysts argue that Obama should use a credible threat of military action, or military action itself, to compel Assad to the negotiating table, just as that action compelled Assad to volunteer his chemical weapons. More dovish analysts would say that strikes should be off the table in both cases. Others might suggest that the goal of ending the war is simply out of Obama's reach and that, if he were brutally honest, he would drop it from his speeches. Those are all reasonable and internally consistent cases. But perhaps they're not what Obama believes can sell at the United Nations this year.