LONDON – President Obama and President Dmitri A. Medvedev of Russia, in their first meeting, vowed a ‘fresh start in relations and announced their intention to cooperate on a variety of issues, beginning with negotiations on a new arms control treaty. In seeking to recast a relationship that has been teetering on the brink of a new cold war, the two leaders also promised to work together on the war in Afghanistan and efforts to rein in Iran’s nuclear ambitions. Appearing after a 70-minute session here, the two struck a warm tone. ‘What we’re seeing today is the beginning of new progress in the U.S.-Russian relations, Mr. Obama said. ‘And I think that President Medvedev’s leadership is, and has been, critical in allowing that progress to take place. The relationship has suffered in recent years over a series of issues, from missile defense to NATO expansion to Russia’s invasion of the Georgian territory of South Ossetia. Mr. Obama conceded that there remained ‘real differences between the countries. But he said he had no intention of ‘papering over those differences, which he said had developed because ‘the relationship between our two countries has been allowed to drift. […]