As he laid out his strategy to combat the Islamic State in both Iraq and Syria, President Obama rejected the “best military advice” of his top military commander in the Middle East.

Quoting two U.S. military officials, the Washington Post reported on Wednesday that Army Gen. Lloyd Austin, commander of U.S. Central Command (CENTCOM), said “that his best military advice was to send a modest contingent of American troops, principally Special Operations forces, to advise and assist Iraqi army units in fighting the militants.”

Austin’s recommendation was taken to the White House by the chairman of the Joint Chiefs of Staff, Gen. Martin Dempsey. The White House rejected CENTCOM’s “advise and assist” contingent due to concerns about placing U.S. ground forces in a frontline role.

In a press briefing Thursday, White House press secretary Josh Earnest said that the president had rejected Austin’s recommendation because he believes “it is not in the best interest of American national security to send American combat troops in a combat operation to act on the ground in Iraq.”

In a nationally-televised speech on Wednesday evening, President Obama repeatedly emphasized that U.S. forces will not have a combat role in Iraq. “We will not get dragged into another ground war in Iraq,” the president said. He specifically underscored that “this effort will be different from the wars in Iraq and Afghanistan,” and will resemble U.S. counterterrorism campaigns in Yemen and Somalia.

Instead, President Obama opted for a more modest course, sending an additional 475 troops to assist Iraqi and ethnic Kurdish forces; 150 of those forces will form more than a dozen teams and embed with Iraqi Security Forces at the brigade level and above, according to the Pentagon. In other words, U.S. advisers are likely to remain inside bases assisting with issues like training, intelligence, and equipment. The remainder will be assigned to conduct intelligence, surveillance, and reconnaissance missions and oversee U.S. military activities at headquarters in Baghdad and Erbil.

Austin’s predecessor, Marine Gen. James Mattis, told the Washington Post that the president’s decision may place the mission at risk. “The American people will once again see us in a war that doesn’t seem to be making progress,” Mattis told the paper. “You’re giving the enemy the initiative for a longer period.”

Supporters of the president’s approach, such as Rep. Adam Smith (D-Wash.), see U.S. combat troops as unnecessary, and could distract the Iraqi government and security forces from taking necessary steps to drive out ISIS militants. “Ranking Member Smith believes combat forces are not necessary in Iraq and would not help. The key is to reform the Iraqi forces and get the Sunnis to turn against ISIL,” said Michael Amato, spokesman for the top Democrat on the House Armed Services Committee.

Opponents of combat troops in Iraq say recent successes show the president’s strategy can succeed. U.S. airstrikes have helped repel ISIS advances on the city of Erbil, and aided Iraqi forces in recapturing the Mosul Dam and the city of Amerli.

But the newest phase of the U.S. campaign against ISIS faces substantial risks, including a dependence on Iraqi political and military leaders.

President Obama conditioned additional U.S. action against ISIS on the formation of an inclusive Iraqi government. Now, his strategy relies on the realization of equally inclusive governance under Prime Minister Haider al-Abadi. The president is counting on the prime minister to make substantial progress in healing sectarian wounds that festered under his predecessor, Nouri al-Maliki. But even if the new Shia-led government is determined to reconcile with Iraq’s Sunni minority, lingering resentment and mistrust could impair efforts to convince Sunni tribesmen to reject ISIS and assist in pushing the militants out of the country.

Militarily, the United States is counting on an Iraqi military with a reputation for retreat to join forces with Kurdish and Shiite militias to wage a ground offensive to recapture territory held by ISIS. Many military experts are skeptical that the Iraqis – with ineffective military leadership and sectarian divisions throughout their ranks – will be able to defeat determined and ruthless ISIS militants without the kind of American military assistance the president has ruled out to date.

The president’s strict reliance on air power also carries risks. When the United States took on al-Qaeda in Iraq (AQI) during “the Surge,” the strategy included special operations forces, conventional units, and intelligence operatives on the ground. Those elements are absent from President Obama’s strategy, despite the fact that ISIS is arguably a more powerful enemy than AQI in terms of manpower, weaponry, financial resources, and territory.

The difficulties of relying on airpower are likely to present themselves as U.S. and Iraqi forces attempt to dislodge ISIS militants from major urban centers. In cities like Mosul, Fallujah, and Ramadi, ISIS can adopt a more covert, insurgency-style approach blending in with local populations. In such an environment, skilled ground troops will be required to sort out enemy forces and remove them block by block.

If Iraqi and Kurdish forces prove unable to carry out such operations and progress against ISIS stalls, would the White House reconsider embedding U.S. special operations forces with frontline Iraqi units to advise and assist?

White House press secretary Josh Earnest delivered a mixed message on that question Thursday. President Obama “is not contemplating deploying additional combat troops on the ground in either Iraq or Syria,” Earnest told reporters. But when asked if the president remains open to mission-specific applications of special operations forces if the need arises, Earnest said he was “not willing to broadly take anything off the table.”