W hen Assimi Goïta, the leader of Mali’s military junta, sat down with the Russian president, Vladimir Putin , in the Kremlin last summer, it symbolised Moscow’s commanding sway over Mali at the expense of the west. As the two men spoke, roughly 3,500 miles to the south, about 2,000 Russian troops were propping up the regime in the landlocked desert country, as part of Moscow’s broader push for influence across the Sahel region. But in the last few days, a wave of coordinated, surprise attacks by jihadist militants and a separatist group has exposed the limits of Moscow’s reach and military might in the impoverished west African state. Over the weekend, rebel fighters launched one of their most effective assaults in years against the Russian-backed authorities. Fighting continued into Monday, with the full picture still unclear. The rebels have so far achieved at least one major victory.…