Wars are a lot easier to get into than out of. Those who ask about exit strategies or question what will happen if assumptions prove wrong are rarely welcome at the conference table when the fire-breathers are demanding that we strike—as they did when advocating invading Iraq, intervening in Libya and Syria, or bombing Iran's nuclear sites. But in recent decades, presidents confronted with tough problems abroad have too often been too quick to reach for a gun. Our foreign and national security policy has become too militarized, the use of force too easy for presidents.

Today, too many ideologues call for U.S. force as the first option rather than a last resort. On the left, we hear about the "responsibility to protect" civilians to justify military intervention in Libya, Syria, Sudan and elsewhere. On the right, the failure to strike Syria or Iran is deemed an abdication of U.S. leadership. And so the rest of the world sees the U.S. as a militaristic country quick to launch planes, cruise missiles and drones deep into sovereign countries or ungoverned spaces. There are limits to what even the strongest and greatest nation on Earth can do—and not every outrage, act of aggression, oppression or crisis should elicit a U.S. military response.

This is particularly worth remembering as technology changes the face of war. A button is pushed in Nevada, and seconds later a pickup truck explodes in Mosul. A bomb destroys the targeted house on the right and leaves the one on the left intact. For too many people—including defense "experts," members of Congress, executive branch officials and ordinary citizens—war has become a kind of videogame or action movie: bloodless, painless and odorless. But my years at the Pentagon left me even more skeptical of systems analysis, computer models, game theories or doctrines that suggest that war is anything other than tragic, inefficient and uncertain.