Obama arrived late for the session, a lot of people think he did on purpose.
According to Biz Pac Review:
A video posted to Youtube shows the seat beside Turkish President Recep Tayyip Erdogan, reserved for the American president, was empty as Erdogan called the group to observe a moment of silence.
In the video, the placard to the right of the Turkish leader was clearly labeled “United States” in front of a very empty chair. German Chancellor Angela Merkel was shown looking over her shoulder, as if to see where the missing Obama might be.
Despite being late to the session, later in the day Obama tried to spin the most heinous loss of civilian life in Paris since Hitler as a mere “setback” in the war against the Islamic State.