By Jim Ribble, CNN
(CNN) – President Obama met this week with Vladimir Putin at the G20 summit in Mexico. To find out more about the dynamic between the two world leaders, CNN's Jim Ribble spoke with Clifford Gaddy, a senior fellow at the Brookings Institution. Gaddy is just completing a book about Putin. He says Putin respects the power of the United States, but his KGB background makes him judge leaders by their potential for vulnerability.