President Barack Obama faces growing skepticism in Washington over his plan to defeat Islamic State jihadists in Iraq and Syria without US ground troops, with critics blasting the strategy as unrealistic and half-hearted.

Obama’s repeated vow that he will not order “boots on the ground” has infuriated Republicans in Congress, who argue that air power alone cannot roll back the IS extremists given the absence of a viable moderate rebel force in Syria and the Iraqi army’s poor track record.

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Their view has been reinforced by former US commanders who question the idea of openly ruling out the use of ground troops, saying it could signal weakness to both adversaries and allies.

Retired Marine general James Mattis, the blunt-talking former head of US Central Command, told lawmakers Thursday you should not “tell your adversary in advance what you are not going to do.”

“The fewer restrictions we place on ourselves going into this, the more apt we are to see other nations give their full measure,” Mattis told lawmakers.

The Obama administration for its part has sent out mixed messages at times about how it will achieve its goal to “destroy” the IS group, and top deputies have disagreed in the past over whether to intervene in Syria.

The chairman of the US Joint Chiefs of Staff, General Martin Dempsey, who acts as the president’s top military adviser, suggested this week the prohibition on ground troops might not always apply as some “advisers” in Iraq may have to enter into combat to ensure air strikes hit their targets.

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The White House quickly sought to downplay his remarks and insisted the plan remained to defeat IS with American fighter jets in the air and US-armed local forces on the ground.

Pentagon chief Chuck Hagel and Secretary of State John Kerry faced a tough reception as they tried to defend the war strategy at hearings Thursday, with lawmakers on the right dismissing Obama’s plan as timid and liberals on the left warning of another disastrous, unwinnable war in Iraq.

Republican Senator Bob Corker asked how the “feckless” moderate opposition in Syria could form the pillar of America’s “entire ground game,” while Senator John McCain said Obama had undercut his effort by ruling out the option of using ground forces.

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“I think that by continuing to repeat that, that the US won’t put boots on the ground, the president in effect traps himself,” McCain said.

The senator, who has long argued for intervention in Syria, said the White House plan was wrong-headed as it failed to offer an incentive to rebel forces there, whose ultimate enemy is President Bashar al-Assad’s regime — not the IS group.

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“You cannot ask people to go and fight and die unless you’ve promised them that you will defeat their enemy, and defeat them right away. You can’t say wait until we defeat” IS, McCain said.

The concerns about the Obama administration’s approach extend to some of America’s allies, particularly Arab governments in a newly-formed anti-IS coalition, analysts said.

Arab countries question whether a moderate Syrian rebel force can be effective, doubt if a Shiite-led Iraqi army can shed its sectarian ways and “wonder about US staying power,” wrote author Aaron David Miller in Foreign Policy.

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– Underlying tensions –

Although the US-led effort could keep the IS group “off balance,” the underlying tensions that feed the jihadist threat stem from deep political and religious divisions that cannot be solved by outside action, said Miller, of the Woodrow Wilson International Center for Scholars.

The international coalition cannot destroy IS, “only Syrians and Iraqis can do that,” he said.

Anthony Cordesman of the Center for Strategic and International Studies, who has advised the Pentagon, acknowledged the strategy represented a gamble but said there were no risk-free options given the sectarian politics that were out of America’s control.

The political debate in Washington was marked by “over-simplified” soundbites and did not reflect the fraught situation, he said.

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“Everyone would like to be able to do this neatly with decisive force,” he told AFP. “But there’s no way to do it in practice.”

If US ground combat troops were sent to Iraq, they “would be an unpopular, non-Islamic force that would inevitably be perceived as taking sides in Iraq’s civil conflict,” he said.

Left-leaning pundit Robert Kuttner of The American Prospect expressed little enthusiasm for the air war and called Obama’s campaign “far from an ideal policy,” but he said, “it’s hard to come up with a better one.”