Delivering his much awaited Nobel speech, the US president said war is sometimes necessary and justified.
Urging the people to accept the 'hard truth' that eradication violence cannot be achieved, Obama said that countries sometimes are forced to wage war to protect their citizens from terrorism and evil regimes.
Further justifying violence, the president said a nonviolent movement could not have stopped Hitler and his regime and similarly negotiations cannot persuade al Qaida to give up terrorism.
The US president also acknowledged the fact that many people questioned whether he deserved the prize or not and also mentioned his recent decision to send 30,000 more US troops to fight in Afghanistan.
Earlier, when he arrived in Osla he defended his Afghanistan policy and said that he was not ready to debate on his plan to initiate withdrawal of troops in 2011.