Western intelligence officers believe Russian President Vladimir Putin may be worried about a possible revolt. But who could actually challenge him? 

Russia will hold parliamentary elections in September, and even in an authoritarian state, elections are a stress test. A flagging economy and a still-not-completed "special operation" in Ukraine could lower his public approval ratings.

Or could he be fearing a threat from within his own circles, from the military's top brass and the oligarch elite?

We ask Sam Greene, professor of Russian politics at King's College London, for his assessment.