The Pentagon is ready to try what appears to be a "Field of Dreams" strategy in Iraq, hoping to entice Sunnis to join the fight against ISIS with an "If you build it, they will come" plan for a new military training site. 

Right now, the U.S. is training mostly Shiite fighters and Kurds, not Sunnis. To date, the Iraqi government has been unable to bring minority Sunnis back into the fold, after they were marginalized by the Shiite-led government. 

With President Obama authorizing up to 450 additional U.S. troops to help in the training effort, White House and Pentagon officials say a major goal will be to bring those Sunnis on board. 

But whether they can is an open question. And Republican lawmakers say the Obama administration still lacks a coherent strategy that can reverse ISIS gains. 

"I don't think 450 troops are going to make the difference," Rep. Adam Kinzinger, R-Ill., told Fox News, while still calling Wednesday's announcement a "good start." 

At the Pentagon, spokesman Col. Steve Warren told reporters the 450 additional U.S. forces would be fully up and running in six-to-eight weeks. The new U.S. troops will go to Taqaddum Air Base in Anbar, in the city of Habbaniyah -- between Ramadi and Fallujah, both under ISIS control. 

A major goal, he said, is to "conduct outreach to Sunni tribes and Sunni militias and to begin bringing those [forces] into the fold." 

But he said U.S. troops would not leave their new base in Anbar unless "certain conditions" are met including a "minimal" chance of contact with the enemy.   

Instead of recruiting outside the base, Warren predicted: "Sunnis will come to them." 

The White House faced skeptical questions on a conference call Wednesday, as even White House Press Secretary Josh Earnest has acknowledged there are more trainers than recruits right now.   

Deputy National Security Adviser Ben Rhodes said Sunnis have not joined the Iraqi security forces due to "political tensions and divisions in Iraqi politics over the last several years." 

Rhodes said, "We are aiming to find with the Iraqi government different ways to tap into that recruitment pool." 

The additional U.S. troops would join the roughly 3,100 American troops already in Iraq, currently training about 3,000 Iraqis. 

Rhodes said the Iraqi government, with the coalition, is reaching out to the Sunni tribes to "try to create a space for them to be a part of the popular mobilization forces that are emerging in different parts of Iraq." This effort, he predicted, would "provide for a greater recruitment base for fighters to both get in the fight against ISIL, but also receive training, equipping and assistance from the coalition." 

It's an ambitious goal. And Republican critics say it's still not enough. 

In a scathing statement, Sen. John McCain, R-Ariz., said the 450 U.S. troops would respond to a "real need" but he remains "deeply concerned that this new deployment is disconnected from any coherent strategy to defeat ISIL." 

He also reiterated concerns that the forces will not deploy closer to the frontlines to help call in airstrikes and advise smaller units. 

"Nearly 75 percent of U.S. strike missions return without firing a weapon because we do not have a unified air-ground campaign," he said. 

State Department spokesman John Kirby, though, told Fox News on Thursday that the U.S. is "very good" at these kind of training campaigns, and vowed that ISIS will be defeated. 

Fox News' Lucas Tomlinson contributed to this report.