President Obama has gotten defensive over his decision to conduct airstrikes in Iraq and provide weapons to allied factions there but not in Syria.

He reportedly dismissed congressional criticism of his Syria approach as "horse-expletive." The idea that arming the Syrian rebels would have deterred Syrian strongman Bashar Assad is a "fantasy," Obama said. And he has been forced to assuage liberal concerns about another extended quagmire in Iraq.

But even prominent supporters, most notably Hillary Clinton, say Obama was forced to reluctantly wade into Iraq again because of his decision to not get more involved in Syria — which allowed the Islamic State to make rapid gains in the region.

The president is now left to explain why the crisis in Iraq prompts a major U.S. response while insisting it would be counterproductive to devote resources to neighboring Syria, where the violence has been even more devastating.

Here are Obama’s attempts to explain his dueling actions — and rebuttals to his blueprint for how to address Iraq and Syria.

Supplying arms to Syrians won’t work

Obama, over the objections of many in his national security team, argues that giving weapons to opposition forces would not reduce Assad’s stronghold on Syria — and could perhaps make the situation worse.

“This idea that we could provide some light arms or even more sophisticated arms to what was essentially an opposition made up of former doctors, farmers, pharmacists and so forth, and that they were going to be able to battle not only a well-armed state but also a well-armed state backed by Russia, backed by Iran, a battle-hardened Hezbollah, that was never in the cards," he told the New York Times in a recent interview.

But with Clinton, his former secretary of state, publicly questioning that logic, Obama’s strategy is being questioned with a newfound intensity.

"The failure to help build up a credible fighting force of the people who were the originators of the protests against Assad — there were Islamists, there were secularists, there was everything in the middle — the failure to do that left a big vacuum, which the jihadists have now filled,” Clinton told the Atlantic.

At the heart of the debate is whether the U.S. can trust the people receiving such weapons. Obama has concluded that Islamic State sympathizers would be among those controlling such arms in Syria. But he believes the Kurds have a real chance to weaken terrorist cells in Iraq.

Limited airstrikes in Syria won’t make a big difference

The president famously decided to pursue airstrikes in Syria before changing his mind and asking for congressional authorization to do so. Lawmakers never united behind a plan, and Obama has made clear that he won't pursue such an option again.

The main difference between Iraq and Syria, Obama says, is that the campaign in Iraq is more limited and that bombings are directly protecting American forces.

“We haven’t had a situation in Syria where U.S. personnel are being directly threatened,” said Jordan Tama, an intelligence and counterterrorism policy adviser to Obama's 2008 presidential campaign. "Generally, the situation has been one where the U.S can’t save 40,000 Syrian lives by a few airstrikes. It would require a much more robust military intervention.”

The airstrikes in Iraq are devoted to securing the Kurdish region, Obama's allies argue. In Syria, such strikes against the Islamic State aren’t so narrow in scope and could actually strengthen Assad’s standing in the war-torn nation, they say.

The White House also argues that Iraq has a political framework to support the U.S. mission, whereas there is no faction in the Syrian government to strengthen American military efforts there.

Still, even the president has shot down suggestions that U.S. forces can swiftly change the situation on the ground in Iraq.

“I don’t think we’re going to solve this problem in weeks,” Obama said ahead of departing for his vacation in Martha’s Vineyard. “I think this is going to take some time. “

And Republicans say Obama is being naive about the threat the Islamic State presents to the U.S.

“It is far past time for President Obama to develop a comprehensive strategy to combat the threat posed by [the Islamic State],” said Sens. John McCain, R-Ariz., and Lindsey Graham, R-S.C., on Tuesday. “While the humanitarian aid efforts undertaken by the administration are an important first step, they should be accompanied by additional steps to degrade [the Islamic State's] capabilities, including U.S. airstrikes against [the Islamic State's] positions in both Iraq and Syria and the immediate provision of military assistance to our partners who are fighting against [the Islamic State]."

Western allies weren’t on board with military intervention in Syria

Major U.S. partners, at least thus far, have expressed support for Obama’s humanitarian mission in Iraq and even his use of airstrikes. That’s a major difference from how the Syria debate played out, when, for example, British lawmakers blocked Prime Minister David Cameron’s bid for military strikes to help the Syrian opposition.

“This isn’t something we’re going alone on,” a senior administration official told the Washington Examiner. “It strengthens the cause. That’s invaluable.”

The American public was opposed to an extended U.S. campaign in Syria

Since the Syrian civil war began, Americans have been opposed to using airstrikes in the Middle Eastern nation. At the same time, however, they have said Obama is handling Syria poorly.

The White House is well aware that the public would grow even more skeptical of the president if Obama authorized military action in Syria. In other words, it’s a political loser.

The question for the White House is whether voters will view the latest round of airstrikes as an extension of the massively unpopular Iraq War. If they do, Obama may be less inclined to continue bombings in Iraq as the November midterm elections approach.