For more than 50 years, the world’s two largest nuclear superpowers — the U.S. and Russia — have agreed to rules limiting their nuclear arsenals. That arrangement ended on Thursday. Our national security correspondent David E. Sanger explains how we got here. Video by David E. Sanger, Coleman Lowndes, Nikolay Nikolov, Alexandra Ostasiewicz, Thomas Vollkommer, Laura Salaberry and Whitney Shefte/The New York Times #russia #trump #nuclear #treaty