In an interview on CNN Thursday, Allen said locals in parts of the world that give birth to such groups "understand the region better." The United States, he continued, should "work closely with them" to resolve the underlying conditions which are fueling recruitment to Islamist factions.
Allen did not identify any causes he personally believes the United States need to address.
He did say he thinks the United States should "consider" establishing a no-fly zone in Syria, but admitted the move way be too costly in resources, Antiwar.com reported. The State Department recently indicated such considerations are off the table because of the cost.
In September, Allen informed the administration of his intention to resign. He cited growing frustration with the administration's policies in the war, and in particularly in not following through on escalations of the conflict, Antiwar.com reported.
He had been warning that he believed the war would last "a generation or more" since last summer.
Allen said he would not rule out returning to a government position like Secretary of Defense in the future, saying he is always willing to "come off the bench if they need the help."
President Barack Obama last month appointed Brett McGurk, a State Department veteran and Allen's former deputy, to replace the outgoing general.
