Strategic ChallengesSince 2001, the United States has endured a tumultuous period, one dominated by the 9 11 attacks and all that has followed: the war on terrorism, the Afghan and Iraqi campaigns, looming confrontations with known or suspected proliferators of weapons of mass destruction, and episodic explosions of mass violence in chronically unstable regions. In this second half of the decade, these and related strategic challenges will test the skill, tenacity, and