Secretary of Defense James Mattis yesterday urged combatants in Yemen, including Saudi Arabia and Yemen’s Iran-backed Houthi faction, to negotiate a cease-fire in that war within 30 days while speaking to diplomats, military officers and conflict-resolution specialists at the U.S. Institute of Peace. In a webcast conversation moderated by former national security advisor and USIP Chair Stephen J. Hadley, Mattis also discussed global security challenges facing the United States—from Russia and China, to North Korea—cybersecurity and the need for the developed world to help fragile states improve their governance and address the root causes of extremism.