US Defense Secretary Ash Carter recently visited Berlin to assure allies that the US would deter aggression. NATO leaders are worried that Russia might invade the Baltics in a Crimea-style fait accompli, and then threaten nuclear escalation unless the alliance backs down.

Moscow's treaty violations and "nuclear sabre rattling," Carter warned, raise "questions about Russia's commitment to strategic stability" and to "the profound caution that world leaders in the nuclear age have shown over decades to the brandishing of nuclear weapons."

This is but the latest confirmation that we've entered a new nuclear age — one characterized by different rules, more actors, less predictability and the paradox that America's conventional superiority may make deterrence harder.

After noting that opponents might be tempted to employ nuclear weapons to overcome conventional inferiority, the 2014 Quadrennial Defense Review observed that US nuclear forces should deter nuclear-armed adversaries from escalating their way out of failed conventional aggression.

"Escalate to de-escalate" tactics have already been publicly embraced by Russia but could also be used by North Korea or China. Instead of graduated rungs along an "escalation ladder," adversaries may well be tempted to lower their nuclear thresholds to forestall conventional defeat.

Last November, then-Defense Secretary Chuck Hagel called nuclear deterrence the department's "highest priority mission." But it is official US policy to reduce reliance on nuclear weapons and pursue a world without nuclear weapons. This may weaken nuclear deterrence because allies and adversaries will wonder how the US might respond to limited nuclear employment.

Plotting to offset US conventional superiority has prompted some states, like North Korea and Iran, to pursue nuclear weapons, and others, like Russia, to increase their reliance on nuclear weapons.

To keep the nuclear threshold elevated in the minds of potential adversaries, the US must have more flexible and credible means to control escalation. The distinction between strategic and nonstrategic nuclear weapons is long obsolete. Any use of a nuclear weapon could have profound strategic effects.

In a new report, "Project Atom," we recommend that in addition to retaining our traditional strategic deterrent, the US needs to acquire nuclear capabilities that enable it to respond proportionately to employment of a nuclear weapon. Specifically, the US should develop options for more forward-deployed assets and more discriminate weapons.

Proliferation by Iran or others could strain extended deterrence and invite allies to re-evaluate their non-nuclear status. During the Cold War, large-scale conventional aggression was not deterred by US or NATO declaratory policy, but by the significant presence of nuclear weapons in Europe and the Pacific. Establishing credibility may require greater nuclear burden-sharing and forward-basing.

Nuclear submarines and ICBMs should remain the highly survivable foundation of US deterrence. Dual-capable F-35s on land and aboard carriers would provide forward-based or rapidly deployable aircraft. Penetrating bombers remain a visible complement to both missions.

More discriminate weapons may be needed. The future B61 gravity bomb will retain lower-yield options and no longer require a parachute for delivery, catching up to 1990s JDAM-like guidance. Credibility would be further enhanced through low-yield weapons deliverable across the triad, as well as additional nuclear-capable standoff cruise missiles from air, sea and land.

But new thinking from Washington is also required. Both statutory restrictions and policy limitations prevent the US from developing new weapons, components, missions or capabilities. The average weapon in today's stockpile is over 28 years old. Current modernization plans will further limit options, since there is no path to replace the B61-11 earth penetrator. In the near term, the national laboratories could be freed to begin researching new designs for lower cost; more safety, security and reliability; lower yields; and other effects.

After a long procurement holiday, the US deterrent is now entering a bow wave of investment and recapitalization. Over the next two decades, a new set of post-Cold War delivery systems will be built, and many of today's weapons will be life-extended. Infrastructure modernization is also badly overdue; uranium facilities in Tennessee, for instance, date to the Manhattan Project.

Current modernization plans are critical just to retain current capabilities, and avoid disarmament by rust. While requiring 3 to 6 percent of the defense budget over the next decade, these investments should be made with an eye to future geostrategic realities.

Broadening options available to a president would strengthen US extended deterrence, discourage proliferation among allies and communicate that there are no potential gaps for adversaries to exploit. This is not about "war fighting" or making weapons "more usable," but making deterrence more credible. Failure to adapt to new realities could invite nuclear use by creating false perceptions that the US would be self-deterred.

Our conventional superiority tempts our adversaries into lowering their nuclear thresholds. A newer, more flexible and more credible US nuclear deterrent designed for 21st century challenges would raise that threshold and help make nuclear employment less attractive.

Clark Murdock is a senior adviser and Thomas Karako is a senior fellow at the Center for Strategic and International Studies. They are respectively the lead and contributing authors of the "Project Atom" report.