President Barack Obama gathered with world leaders in a day of delicate diplomacy, as he sought to rally the international community on Monday around efforts to isolate Russia following its incursion into Ukraine. Nuclear terrorism was the official topic as Obama and other world leaders streamed in to a convention center in The Hague for a two-day nuclear summit. But the real focus was on a hurriedly scheduled meeting of the Group of Seven industrialized economies to address the crisis in Ukraine on the sidelines of the nuclear summit.