Since the end of World War II, we have seen the emergence of a “liberal world order.” By any historical standard, it’s brought us extraordinary peace, prosperity and progress. Though imperfect, it’s preferable to any other option currently available. But unless the U.S. defends it and invests in it, it will die—sooner rather than later. That, in a nutshell, is the argument Robert Kagan makes in his powerful new book: The Jungle Grows Back: America and Our Imperiled World. He joins FDD president and Foreign Podicy host Clifford D. May for a discussion of what human progress in the 21st century requires.