By Mac Thornberry and Andrew F. Krepinevich Jr.

The next U.S. president will inherit a security environment in which the United States con­fronts mounting threats with increasingly constrained resources, diminished stature, and growing uncertainty both at home and abroad over its willingness to protect its friends and its interests. Revisionist powers in Europe, the western Pacific, and the Persian Gulf—three regions long considered by both Democratic and Republican administrations to be vital to U.S. national security—are seeking to overturn the rules-based international order. In Europe, Russian President Vladimir Putin has seized Crimea, waged proxy warfare in eastern Ukraine, and threatened NATO allies on Russia’s periphery. Further demonstrating its newfound assertiveness, Russia has dispatched forces to Syria and strength­ened its nuclear arsenal. After a failed attempt to “reset” relations with Moscow, U.S. President Barack Obama has issued stern warnings and imposed economic sanctions, but these have done little to deter Putin.

Nor has the administration’s “pivot” to Asia, now five years on, been matched by effective action. China continues to ramp up its military spending, investing heavily in weapons systems designed to threaten U.S. forces in the western Pacific. As a result, it is proving increas­ingly willing and able to advance its expansive territorial claims in the East China and South China Seas. Not content to resolve its disputes through diplomacy, Beijing has militarized them, building bases on natural and artificially created islands. The United States has failed to respond vigorously to these provocations, causing allies to question its willingness to meet its long-standing security commitments…

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