One of America's most respected former military officials is worried about the 2016 presidential field

Robert Gates hesitated, considering the once out-of-this-world possibility that has become more and more plausible lately.

"Could you imagine a President Donald Trump?" he was asked.

"In all honesty, that's difficult for me," he told Business Insider in an interview earlier this week.

In fact, the entire 2016 presidential field seems to be confounding Gates, one of the most-respected military officials in modern US history.

Gates served as the US secretary of defense under Presidents George W. Bush and Barack Obama, and is the only person to have held that position under two presidents of two different political parties.

Yet just more than a week before the first votes are cast in the presidential primaries, none of the current crop of candidates, Gates told Business Insider, stands out as especially presidential.

Hillary Clinton, the Democratic front-runner with whom he worked in the Obama administration, does not have his endorsement. Some of Trump's rhetoric is not helpful. Sen. Ted Cruz (R-Texas), Trump's main GOP rival, doesn't know what he's talking about.

"People are making a lot of grandiose promises and pledges to do one thing or another, either on domestic affairs or on foreign policy," said Gates, who is promoting his new book, "A Passion for Leadership."

"But I haven't heard anybody really talking about how they would fix some of the problems that they're describing and how they would actually implement them," he added.

Gates went on to take an apparent swipe at Trump, who has proposed to build a wall along the US-Mexico border and "make Mexico pay for it."

"Some of the rhetoric in terms of how they would deal with problems is frankly totally unrealistic, in terms of making other countries do one thing or another, and so on," Gates told Business Insider.

"So what we're hearing," he added, "is people competing with each other to make ever-broader and more fantastic promises about all the things they're going to change that, in my experience, are completely incompatible with reality."

Gates has been struck by the phenomenon of both Trump and Sen. Bernie Sanders (I-Vermont), who has risen as an insurgent challenger to Clinton on the Democratic side.

But he took issue with what he called Trump's "over-the-top" rhetoric, particularly on foreign policy. Trump frequently professes at campaign rallies that his administration would "bomb the s--- out of" the Islamic State terrorist group, also known as ISIS. And Trump has been subject to intense criticism for a plan to temporarily bar Muslim tourists and immigrants from entering the country.

"How he intends to deal with issues is pretty unrealistic," Gates said of Trump.

Gates had perhaps even harsher criticism later in the week for Cruz, who has emerged as Trump's main threat on the Republican side. Cruz last year called for "carpet bombing" the Islamic State, an indiscriminate strategy that critics say would lead to large numbers of civilian casualties.

In a Tuesday "Morning Joe" interview, Gates said the tactic would be "totally useless" and end up backfiring on decades of US war strategy.

"Carpet bombing would be completely useless. It's totally contrary to the American way of war," Gates said. "Total disregard for civilians. So, I mean, part of the concern that I have with the campaign, particularly when it comes to national security, is that the solutions being offered are so simplistic and so at odds with the reality of the rest of the world, with the way the world really works."

Indeed, Gates took aim at some of the supposed fantastical prescriptions to ISIS in a round-robin criticism of the various GOP candidates.

Sending 10,000 troops into Iraq and Syria to combat the group, as Sen. Lindsey Graham (R-South Carolina) and former Sen. Rick Santorum (R-Pennsylvania) have suggested, is "unrealistic," he said. So too is the idea of bringing in troops from other Sunni Muslim nations into Iraq, which is run by a Shiite-influenced government.

Gates described the most "workable" strategy as an amped-up version of Obama's current war plan - which includes special-operations forces in Syria and US military advisers helping to train Sunni tribes, Iraqi Kurds, and the Iraqi Security Forces.

"But I think what most candidates don't want to tell the American people is the reality is this a problem that is going to take some time to resolve," he said.

As for one of the officials with whom he worked in the Obama administration - Clinton - Gates had less glowing words than a 2014 interview in which he said he thought Clinton would make a good president.

"I don't think I ever said that," he told Business Insider.

Gates called Clinton's decision to use a private email server during her time in the State Department a "mistake" - a word she has also used. Later in the week, he also told radio host Hugh Hewitt that he agreed with former CIA Director Michael Morell's claim that Clinton's private server had likely been hacked by foreign governments.

"I actually never used email for official purpose - even the government servers," he told Business Insider. "If I was going to direct somebody to use military force or undertake a covert action, I wanted to do it face to face. And I wanted my signature to be on a piece of paper."