For the first time in decades, the U.S. is tightening its belt on defense spending. While traditional threats like nuclear and great power conflicts do remain, the post-9/11 challenges of terrorism and counterinsurgency have led to a paradigm shift in the way we think about our national security. Emerging threats like cybersecurity and biowarfare also require new thinking. Do 21st century challenges now pose a greater threat to U.S. national security than traditional threats like nuclear war, naval supremacy and ability to fight ground wars? Defense in an age of economic uncertainty.