US president Barack Obama has arrived in Moscow for a summit with the Russian leader Dmitry Medvedev.It is Obama’s first trip to Moscow since taking office. The two leaders are expected to announce a framework deal on arms cuts. There may also be a key breakthrough on Afghanistan. Moscow may give permission for the US to move weapons, and perhaps troops, across Russian territory and airspace en route to Afghanistan. There are differences, however, on warhead deployment and Russia objects to US missile defence plans in central Europe.