World leaders at the G20 Summit in Turkey have said efforts to combat terrorism must be intensified after the tragic events in Paris - the latest in a series of attacks claimed by the so-called Islamic State. On Sunday, France launched massive air strikes on IS strongholds in Syria. HARDtalk speaks to retired US admiral James Stavridis, who was NATO's Supreme Allied Commander in Europe until 2013. He believes there should be 'meaningful action on a significant scale' by NATO. But are there limits to what military operations can achieve in fighting terrorism?