Seth Cropsey writes: The U.S. has emerged into a new world. To the potential for nuclear warfare with a would-be peer competitor—China—that American statesmen most wished to avoid after the Cold War have been added threats from a nuclear-armed Russia, North Korea, and sooner or later, Iran. The more immediate prospect of a triple hegemony may not be an existential threat. But its outcome would cripple our markets, destroy our alliances, and endanger us where we live. All can be avoided by a change in grand strategy that continues to hold threats at a distance as it relies on the independence, accessibility, and technological superiority of seapower.