(CNN)Secretary of State John Kerry continued his defense Thursday night of the U.S. strategy in Syria, as well as, the Iran nuclear deal, this time advocating for the Obama administration to "Late Show" host Stephen Colbert.
John Kerry: 'Catastrophic mistake' for Russia to back Assad
Colbert started his interview with a question on the minds of many officials in Washington, challenging Kerry, "What is the plan in Syria, and as a follow up question, is there a plan in Syria?"
Kerry responded, "Both questions are fair. Yes and yes. The plan is that we are going to continue to put pressure on ISIL. We've made it clear to Russia, we will continue to fly, we will continue to do what we've been doing for a year."
Kerry also affirmed the need to destroy ISIL while continuing to press for the removal of Syrian President Bashar al Assad. "
"If they're there to support the Assad regime, Russia has made a catastrophic mistake because they will be siding with Assad, with Iran, and with Hezbollah against the entire rest of the community in that part of the world," he said. And on Russian pressure for the U.S. to leave Syrian airspace, Kerry held firm, saying, "No matter what, we are going to continue our policy -- it is our policy -- of destroying ISIL."
But Kerry also appeared to echo a subtle shift in administration rhetoric on the removal of Assad, and instead of calling for his immediate resignation, he suggested a more gradual transition.
"The only way in the end that [the crisis] will be solved is for Assad to transition out over a period of time in a structured, managed way," Kerry argued.
Colbert also asked the Secretary of State about the nuclear deal with Iran, sealed last month by a successful Democratic filibuster in the Senate. The host joked, "Congratulations, I assume -- I don't know. Some people say, you know, you saved us. Some people say you've killed us. Which do you think it is?"
Kerry forcefully defended the deal, saying the agreement was a means of avoiding war with Iran over its nuclear ambitions. He related the decision to his experience in Vietnam.
"What I learned in Vietnam was pretty simple," Kerry said. "It was a war in which too many of my friends died and died, I believed, under circumstances that might have been averted had different choices been made by people in positions of responsibility," he said, adding later that he vowed "that if I was ever one of those people in a position of responsibility, I would do everything in my power to make sure that young men and women in America were never sent to war unless it was a last resort."