A retired lieutenant general rumored to be under consideration to serve as Donald Trump's vice president thinks that the Manhattan billionaire is smart to keep his plans for defeating ISIS to himself.

Michael Flynn, the former head of the Defense Intelligence Agency who spent years fighting terrorist groups in Iraq and Afghanistan, told Business Insider that the US shouldn't show its cards unnecessarily.

Flynn said:

"What we are talking about here is options. Giving our country options against any enemy that we face. ... We want certainly the threats around the world, those that don't like our way of life, we want them to wonder what options we still have."

Trump, the presumptive Republican presidential nominee, has been criticized for being vague about his plans to fight terrorism. Some critics have speculated that he doesn't really have a plan, but Trump says that he just wants to maintain the element of surprise.

Flynn said:

"What he's really talking about, and I'm speaking for Mike Flynn, not Donald Trump, is that he's saying essentially we have to have options. We have to have a lot of options. And, frankly, we do. We do have a lot of options. But we just don't apply them right now in any sort of coherent manner."

He also advocated against drawing so-called red lines, like US President Barack Obama did in Syria when he said that he wouldn't tolerate the use of chemical weapons from the regime of Syrian President Bashar Assad.

When evidence surfaced of Assad's use of chemical weapons, Obama stepped back from his red line and declined to strike, instead allowing a deal with Russia to go through that saw Assad agreeing to give up his chemical weapons.

Flynn said:

"We cannot say, 'Well, this is my red line, if you use chemical weapons, we're going to go in and we're going to go after you.' [A]nd we blinked. We blinked in a big way. And that takes away a lot of the shine of the United States of America."

Flynn is reportedly on Trump's short list for vice president, but it's unclear if he will end up being the pick for running mate. He recently wrote a book: "The Field of Fight: How We Can Win the Global War Against Radical Islam and Its Allies."