Heads of state and government attend the North Atlantic Council meeting in Chicago during the NATO 2012 Summit on May 20, 2012. [Xinhua]

 U.S. President Barack Obama (R) and NATO Secretary-General Anders Fogh Rasmussen (L) welcome French President Hollande on his arrival to the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 U.S. President Barack Obama (R) welcomes German Chancellor Angela Merkel on her arrival to the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 U.S. President Barack Obama (R) and NATO Secretary-General Anders Fogh Rasmussen (L) welcome British Prime Minister David Cameron on his arrival to the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 British Prime Minister David Cameron (2nd L) attends the North Atlantic Council meeting in Chicago during the NATO 2012 Summit on May 20, 2012. [Xinhua]

French President Hollande (2nd R) attends the North Atlantic Council meeting in Chicago during the NATO 2012 Summit on May 20, 2012. [Xinhua] 

 U.S. President Barack Obama (R) and NATO Secretary-General Anders Fogh Rasmussen (L) gesture at the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 U.S. President Barack Obama (R) and NATO Secretary-General Anders Fogh Rasmussen (L) welcome European Commission President Jose Manuel Barroso on his arrival to the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 U.S. President Barack Obama (R) and NATO Secretary-General Anders Fogh Rasmussen (L) welcome Danish Prime Minister Helle Thorning-Schmidt on her arrival to the NATO Summit in Chicago, May 20, 2012. [Xinhua]

 NATO Secretary General Fogh Rasmussen attends the North Atlantic Council meeting in Chicago during the NATO 2012 Summit on May 20, 2012. [Xinhua]