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Vladimir Putin knows how to probe for weakness in the West. With his troops building up on the Ukrainian border, Russia's president is testing the unity of Nato. In particular, he is putting pressure on Europe's eastern flank. How will nations once in the Soviet orbit respond? Stephen Sackur speaks to Kiril Petkov, prime minister of Bulgaria, which Moscow says, must not host a Nato military presence. This is a big test for a new prime minister in the EU's poorest country. Is Bulgaria ready to stand up to Russia?