Former Defense Secretary Robert Gates isn't particularly impressed with the emerging 2016 field despite a wealth of choices, especially on the GOP side.

"I haven't been particularly impressed, frankly, by anybody at this point on either side of the aisle," Gates said on "Face the Nation" Sunday. In particular, he said he wasn't "seeing much courage" for support of trade agreements like the pending Trans-Pacific Partnership.

He also said most of the candidates "have no experience in foreign policy."

"Very few do on the Republican side have any experience, or very little. Maybe two or three years in the Senate. But perhaps their views will be fleshed out, and become more cogent as the campaign goes along," Gates said. "But so far they're in early days, and I think their views are probably largely unformed."

The former defense secretary, who served under both previous Bush presidencies, was critical that likely GOP presidential candidate Jeb Bush stumbled last week when asked whether he would have invaded Iraq in 2003, knowing what we know now about the war.

"That's one question where I would have thought he would have had an answer figured out before he got into the middle of this. It was an inevitable question that would be asked," Gates said. "I think that the way to deal with it, frankly, is to say you don't make policy by going back and reliving old decisions."

By contrast, Gates said, he admired how President George H.W. Bush, George W. Bush and Obama were all decisive leaders who didn't second-guess their decisions.

"They made a decision, and they moved on," he said. "And I think, you know, the same thing as a candidate. You say what you believe, and then you move on."

Bush was initially confronted with the question on Fox News, in an interview that aired Monday. He said he would have authorized the war but later said he misinterpreted the question. After a couple of attempts to answer the question, Bush finally said on Thursday, "Knowing what we now know, I would not have engaged."

Gates was also complimentary of former Secretary of State Hillary Clinton. The two served together in Mr. Obama's cabinet for a few years.

"I think that she was a good secretary of state. I think she played a critical role in getting the much tougher sanctions on Iran. And getting particularly the Russians and the Chinese on board, to allow those more severe sanctions to be put into place," he said.

On the 2009 Afghan troop surge, Gates said, "If anything, she was tougher than I was on that," and that she was ready to support a request from former Gen. Stanley McChrystal for 40,000 troops. The only issue on which he and Clinton disagreed, Gates said, was whether to intervene in Libya in 2011. Clinton supported the airstrikes, and Gates did not.

He did say that some issues about her campaign are "a concern," such as her use of a private email server. As defense secretary, Gates said he used email for personal matters but never for business, calling it "risky."

He also said that there is "an issue of appearances, at a minimum," created by the fact that foreign governments donated to the Clinton Foundation while she was secretary of state.

Asked whether he could see himself voting for Clinton, Gates said, "it's a little early."

"Besides, I'm not sure that having a Republican endorse you is the best thing at this point," he added.

During the interview, he also weighed in on the nuclear deal being negotiated with Iran and the fight against the Islamic State of Iraq and Syria (ISIS).

"Getting the Iranians to the negotiating table in the first place was a success for U.S. foreign policy," Gates said. And while he said that there are some "encouraging" specifics in the framework for the nuclear deal, he still has some concerns including the timing of lifting sanctions and whether investigators will have significant access to all of Iran's nuclear facilities to perform inspections.

He also questioned the entire point of the negotiation.

"I think that the pursuit of the agreement is based on the president's hope that over a ten-year period, with the sanctions being lifted, that the Iranians will become a constructive stakeholder in the international community. That as their economy begins to grow again that they will abandon their ideology, their theology, their revolutionary principles, their meddling in various parts of the region, and frankly I believe that's very unrealistic," he said.

But the alternative is not war, Gates argued. He said alternative is a better deal where the U.S. says, "Here are the additional things we need for this agreement to work, and to be worthwhile."

"If they choose not to come back to the negotiations but to race to a nuclear weapon, well, my guess is that will show that they intended to do that all along, despite all their protestations that they have no interest in a nuclear weapon. But I think that there is a potential for a better deal," he said.

On the fight against ISIS, Gates argued that sending large numbers of U.S. ground troops back into Iraq would be "a serious mistake."

He argued instead for more flexibility for U.S. troops in the region, deeply embedded trainers with Iraqi and Kurdish allies who are carrying out the ground fight, and more forward air controllers and spotters and special forces.

"I think to be able to really get at ISIS we need, and these are relatively small numbers. We have 3,000-plus in country already. It seems to me a fraction of those, if given broader rules of engagement, could play a more effective role," he said.

CBS News Political Reporter Stephanie Condon contributed to this story.