GOP presidential front-runner Donald Trump has been cagey on his foreign policy, saying he doesn't want to tip his hand to America's enemies, but he said Monday he prefers letting Russia fight Islamic State (ISIS) militants in Syria rather than sending American troops.Russia has been building up its forces in Syria recently, and is supporting Syrian President Basher Assad's regime. The United States is siding with the Free Syrian Army in the three-way war in Syria."If Putin goes in there and if he is able to knock out ISIS, which he has a big reason to do it because he doesn't want them coming into other places, if goes in and knocks out ISIS to me that's not the worst thing I have ever heard of," Trump said Monday on Fox News Channel's "On the Record with Greta Van Susteren." The United States is fighting ISIS on its own behalf, but still is helping Assad, Trump said."Assad must think we are the dumbest people on Earth, because ISIS wants to fight Assad but they can't get through because they are fighting us," Trump said."If Russia wants to do the job on ISIS it would be something that I would not be extremely concerned about," Trump said.Van Susteren pressed Trump on his own plans saying that voters deserve to know what their president would do, but, like in the past, he declined."I don't want people to know all my thoughts because I do have a good chance, and if I'm there, I don't want people to go back to your show and say, 'Well, this is what he is going to do,'" he said. "If Gen. Douglas MacArthur and if Gen. (George) Patton ever heard and some of the great generals ever heard all of this talk, this political talk, they would be spinning in their grave."