In in extensive interview with the New York Times posted Saturday, GOP frontrunner Donald Trump outlined his views on foreign policy and said he would be willing as an “absolute last step” to use a nuclear weapon first in an altercation with another country.

“[I]t’s the biggest problem the world has, nuclear capability,” Trump said. “I think it’s the single biggest problem. When people talk global warming, I say the global warming that we have to be careful of is the nuclear global warming.”

Trump spoke to the New York Times about his vision if he was president over the course of two phone interviews for a total of 100 minutes. He also said he would stop buying oil from Saudi Arabia if the country refused to put troops on the ground to fight the Islamic State, that he would remove U.S. security forces from Japan and South Korea if they didn’t contribute more financially for the protection, and that he would not rule out spying on American allies like Germany’s leader Angela Merkel.