Former Sen. Rick Santorum (Pa.) was first up and pointed to potential threats that originate from Latin America. Former Massachusetts Gov. Mitt Romney later agreed with that assessment.
Former Godfather's Pizza chief Herman Cain pointed to cyberattacks, which are getting increased attention from U.S. security agencies.
Former House Speaker Newt Gingrich (Ga.) mentioned several potential threats, including cyber strikes and electromagnetic pulse attacks.
"The pulse can easily span continent-sized areas, and this radiation can affect systems on land, sea, and air," according to the Federation of American Scientists.
Texas Gov. Rick Perry pointed to America's leading global economic and military rival, China. But he then stated his belief that the Asian giant is "headed for the ash heap of history."