In the preamble to the U.S. Constitution, the phrase “provide for the common defense” refers to one of government’s bedrock responsibilities. Simple in concept, complex in execution, it is the subject of extensive debate and today mandates nuclear deterrence.

We are on the precipice of a different world. As Iran pursues nuclear capability, Kim Jong-un rattles his saber while threatening the United States and its regional allies with a nuclear weapon. North Korea’s closest neighbors, South Korea and Japan, are watching closely, knowing we are their shield. They live daily with the threat of an enemy that casually and repeatedly speaks of using nukes and has demonstrated its ability to attack without warning, as it did when its submarine sank the South Korean ship Cheonan on 26 March 2010. Should these countries decide they need their own nuclear weapons, they certainly have the technology and the ability to build them, thereby significantly raising the stakes and the likelihood of war. Enter deterrence.

U.S. nuclear deterrence should:

• Discourage such attacks against the United States by ensuring the ability to deliver an overwhelming response in kind

• Make us resistant to coercion by adversaries using the threat of such an attack to induce concessions

• Reduce the chance of large-scale conventional war between nuclear-armed adversaries because of the shared fear of escalation

• Extend a guarantee to allies and partners, reducing the proliferation of these weapons and, thus, the likelihood of their use

The mission of our nuclear forces is to threaten what an adversary values, thereby averting an attack of this nature on us, our friends, or our regional partners. Deterrence is a matter of perceptions. Its effect takes place in the mind of an opponent living in another country with different values, pressures, and goals.

Its purpose is to influence the other guy’s decision making, not ours. This is so simple, yet so easy to forget. The world’s foremost experts struggle to decipher Kim Jong-un’s thought process, but he may well believe he cannot survive without these weapons. He may feel that they are his source of power. It is possible he believes he holds the ultimate trump card and threatens using nuclear weapons to influence calculations in future international engagements.

Nine countries have this capability. This means conflicts similar to those playing out with North Korea and to a lesser degree Iran will continue to make headlines. The value of our deterrence is that it limits aggressors to threats. They cannot hold us or our partners at risk because we maintain forces that are credible, survivable, and ready. Additionally, our nuclear power may delay hostile action long enough for negotiations to relieve tensions. Kim Jong-un must understand that if he attacks, we will respond with a strong resolve, and that we have an assured second-strike capability.

What is more, our readiness influences China and Russia, both nuclear-capable countries with wide ties and significant sway. The credibility of our nuclear forces and our resolve must remain clear to these great powers as they exert pressure on rogue leaders.

And we must remember that decisions we make now affect the believability of our deterrence later, as we analyze an uncertain future based on what we know today and our best estimate of coming trends. As our nuclear forces reach the end of their lives, we must take action to keep our capability strong and flexible.

President Barack Obama has taken a position consistent with those of many prior administrations. The responsible action now is for us to reaffirm our embrace of a strong nuclear deterrent, thus reducing the chances of proliferation and miscalculation. We must not flinch from this critical test of national character.

Our planned 12 ballistic-missile submarines will provide strategic nuclear deterrence into the 2080s. As we debate the merits of various programs over coming years, we must not lose sight of the fact that they guarantee our very existence, deterring potential enemies from using weapons with unimaginable consequences.