Retired Lieutenant Colonel John Nagl appeared on ABC's This Week and said while Obama's goals in dealing with ISIS are correct, the president has severely underestimated what it is going to take to defeat them.

Nagl is a top army strategist and was a commander on the ground in Iraq. He also co-authored a counter insurgency manual with General David Petraeus. Nagl is responsible for training Iraqi forces but warns that they are "not ready" to handle ISIS and won't be for a while.

His suggested strategy is American special forces -- "A-teams" -- on the ground. That's the key weakness of Obama's strategy, according to Nagl.

Guest host Martha Raddatz spoke with Nagl. Here is an excerpt:

Raddatz: John Nagl, people are already talking about mission creep, and we have, indeed, seen the mission change in Iraq. Originally it was to protect Americans and infrastructure. Now they're going after a much broader group of targets and possibly into Syria. Do you see this as mission creep? It seems like every conflict starts this way, and then it gradually moves to something else.

Nagl: Well, the last Iraq war, of course, didn't start this way. This is going in the other direction. I do think that the goals the president has outlined are correct -- we are at war, that it's going to take a much bigger effort than he has described to date to win this war in any kind of reasonable time to keep the Middle East together and keep Americans safe. So I don't think it's mission creep. I think the president has understated what it's going to take to accomplish the objectives he correctly laid out -- destroying ISIL.