President Asif Ali Zardari has said that the dark forces of extremism, militancy and sectarianism are posing a serious challenge to the integrity of the nation, but no one will be allowed to endanger its existence.The President, in a message on the Defence Day of Pakistan, said that Pakistan was a moderate, tolerant and peaceful nation, where political choices were made by the ballot.“Let us, therefore, pledge that we will never allow anyone to endanger our existence as a moderate, tolerant and peaceful nation,” he said.Stressing an urgent need for clarity of purpose and national unity to confront the challenges, the President said that the demonstration of unprecedented cohesion among the people, singularity of purpose at the national level and valour of the armed forces made September 6, a memorable day in the national calendar.