Editor’s Note: This is adapted from the introductory essay of the latest ISSF Policy Roundtable.

There is no graver threat to the world than nuclear war, a catastrophe some believe is less unlikely than commonly assumed. A president’s most awesome responsibility is control of America’s nuclear weapons. Most presidents, when briefed on the contents on the war plan, are stunned.

As Donald Trump prepares to assume this charge, U.S. nuclear policy finds itself at a historical crossroads. The incoming administration will be presented with at least five tensions that will force difficult choices for the future of American nuclear strategy.

First, the world is in the midst of a technological transformation that threatens to undermine both the idea and the reality of strategic stability based on mutual vulnerability between nuclear-armed states. For decades, advocates of what Robert Jervis called the “nuclear revolution” emphasized the inalterable facts of the atomic age: clean first strikes that eliminated an adversary’s ability to unleash a devastating nuclear response were next to impossible, as were perfect defenses against a nuclear attack. Once a state secured its ability to retaliate to a nuclear strike with forces of its own, there was little point in building more or better nuclear weapons. Seeking nuclear primacy, in other words, was pointless, since greater numbers of nuclear weapons could not change the basic fact of mutual assured destruction. Brendan Green and Austin Long, as well as Daryl Press and Keir Lieber, have conducted trailblazing work to reveal that the United States never fully accepted the premises of mutual vulnerability. In reality, it tried very hard over the years (with varying degrees of success) to escape it. The United States invested extraordinary sums over decades to produce what Press and Leiber term the “counterforce revolution.” Massive improvements in missile accuracy, stealth, and speed, combined with increased sensory and computing power, make today’s nuclear balance far less stable than once thought. Furthermore, as Long highlights, the United States needed to pursue primacy and damage limitation strategies if its extended deterrent relationships with allies were to remain credible.

Second, the same technological revolution that vastly increased counterforce capabilities blurred the once-bright lines between nuclear and non-nuclear weapons. The next president may be able to carry out missions once the sole remit of nuclear weapons with advanced conventional capabilities, such as hypersonic missiles, directed energy, and cyber weapons. This new situation presents its own set of challenges. The United States fights its conventional wars by seeking to blind and cripple its adversary’s command, control, communication, and intelligence capabilities in the opening hours and days of a conflict. As Joshua Rovner points out, this concept of operations may make a nuclear armed target think that the United States seeks to disarm its nuclear response capacity. In such circumstances, that state might feel enormous pressure to escalate the conflict and employ its nuclear weapons. As Long highlights, it is still unclear when and where advanced conventional weapons can substitute for nuclear weapons, especially as adversaries continue to harden, hide, and make mobile their nuclear forces.

Third, the goals of nuclear deterrence and nuclear disarmament are in increasing tension. As Nina Tannenwald reminds us, the non-nuclear weapons states of the world are growing increasingly impatient with the failure of the nuclear weapons states to move toward what are seen as their moral and legal obligations to eliminate their nuclear stockpiles. The humanitarian consequences movement, a globally popular movement barely discussed in the United States, is one reflection of this frustration with the slow pace of nuclear disarmament. A similar tension plays out in the United States, as the Obama administration committed to move towards a world without nuclear weapons while concurrently authorizing a multi-decade, trillion-dollar modernization of American strategic nuclear forces.

Fourth, we continue to witness increased geopolitical strains among states with nuclear weapons. These tensions fall into two categories. The first danger is regional tensions, such as those in East and South Asia (and potentially the Middle East) that are made worse by erratic regimes (think North Korea) or states pursuing aggressive nuclear policies and postures (think Pakistan). Second, geopolitical tensions have increased among the larger nuclear powers. China’s rise has increased the possibility of clashes over disputed maritime claims and territories such as Taiwan while also unnerving Japan and South Korea, who are under the U.S. nuclear umbrella. Russia’s conquest of Crimea, aggressiveness toward NATO’s eastern flank, and loose rhetoric surrounding its own nuclear policy have generated great concern in the United States and Europe. The nuclear shadow hovers menacingly over each of these trouble spots.

Fifth, nuclear policy exposes deep ethical conundrums. On the one hand, a robust nuclear strategy remains a crucial tool of American grand strategy: to deter adversaries, reassure allies, and limit proliferation. On the other hand, short of a highly unlikely “bolt from the blue” nuclear attack on the United States (and perhaps not even then), it remains had to imagine under what scenario the United States would ever detonate a nuclear weapon against an adversary. It is rarely discussed openly, but using these weapons would generate such horrific consequences as to make their employment largely unthinkable. As Long dryly notes:

While any military operation has an ethical component, the vast power of all but the smallest nuclear weapons is likely to produce significant collateral damage if used against targets in any but the most remote and uninhabited locations.

These moral tensions were powerful during the Cold War, when the United States faced an arguably existential threat from a ruthless ideological and geopolitical adversary. Over 70 years since their only wartime use, in a disordered but hardly desperate world, the threat to use unthinkably destructive weapons that lies at the heart of United States nuclear deterrence strategy is both incredible and morally challenging.

At the International Security Studies Forum, we convened five leading thinkers to grapple with these challenges. At one end of the spectrum, Nina Tannenwald recommends that the United States work hard to strengthn the norm of non-use of nuclear weapons. One clear way this could be done is for the United States to embrace a no-first use policy, which was recently debated — and in the end rejected — by the Obama administration. On the other end, Press and Leiber see no way to escape the dynamic technological changes that make the nuclear balance more volatile. Relatedly, Long advocates a continued push for damage limitation capabilities to generate “optimum instability” in order to make U.S. extended deterrence postures credible.

The contributions by Acton and Rovner fall somewhere in between. Acton argues that the current U.S. nuclear strategy of calculated ambiguity is increasingly not credible, while alliance considerations and conventional force imbalances would make a nuclear no-first use pledge unwise at this time. Rovner suggests that the Obama administration successfully threaded the needle between deterrence and what he calls “devaluation.” While full-scale disarmament is, for Rovner, a chimera, careful policies can accomplish both America’s deterrent and nonproliferation missions.

Why is there such a wide range of views and recommendations in the nuclear policy community writ large? Debate and disagreement is not unusual among specialists in American foreign policy and international affairs. The nuclear field, however, seems especially divided and stovepiped into various tribes. Nuclear policy is discussed in fundamentally different ways in different settings. In the academic world, an article on nuclear policy published in a leading political science journal using quantitative methods would have almost nothing in common with a paper published by a historian based on new archival materials, even if the set of questions driving the research were similar. Similar divides and gaps mark the worlds of policy and academics, advocates of deterrence or disarmament, specialists in strategy versus experts in proliferation and non-proliferation, and Americans and everyone else.

Understanding and assessing nuclear policy presents historical and epistemological challenges that are often underappreciated. Analysts make suggestions based on causal claims that are developed by exploring the past. Two problems confront any scholar looking to make arguments based on America’s nuclear history.

The first problem when looking at the nuclear past is getting access to the primary sources, which in many cases remain classified. There is even a greater challenge, however: the existence of at least four distinct and at times competing histories of U.S. nuclear policy since 1945. First, there is the intellectual history of nuclear strategy and strategists — the so-called “wizards of Armageddon” who populated RAND and many university centers in the immediate postwar period. The fascinating story of how these brilliant thinkers developed the language and concepts behind deterrence theory is often mistakenly conflated with actual U.S. policy. The second problem is the rhetorical history of nuclear policy — great speeches or documents, such as Secretary of State John Foster Dulles’s “massive retaliation” speech or Robert McNamara’s presentation to NATO in Athens laying out flexible response doctrine. These documents and speeches were often more important as signals to domestic audiences, allies, and adversaries than as guides to actual war plans. In fact, these private and public declarations were often at odds with the third strand — the operational history of U.S. nuclear weapons. Perhaps the most deeply classified history, the story of what weapons were developed and acquired, how they were deployed and who controlled them, and what strategies were in place for their use, often bore little resemblance to what either game theorists wrote or cabinet secretaries stated (perhaps most perplexing and distressing to a historian). There existed far more continuity in U.S. nuclear war plans across administrations, for example, than public declarations would have indicated. Finally, the most elusive but perhaps most important history is how different U.S. presidents thought about nuclear weapons as a tool to advance American grand strategy, and what views they held on issues ranging from nonproliferation to nuclear deterrence to coercion to actual use.

Perhaps a greater challenge is epistemological in nature. Analysts developing causal claims about nuclear weapons face a welcome problem, as they are trying to understand an event that never happened: why we have never had a thermonuclear war. Explaining why something has never happened is difficult and at best speculative. Many understandably believe that the Soviet Union was deterred from invading Western Europe by U.S. nuclear strategies, but we do not really know for sure. It may well be the Russians never had any interest in attacking the West, whether it fielded nuclear weapons or not. It may also be that nuclear weapons made crises, instability, and war more likely. Indeed, the dangerous (second and third) Berlin and Cuban Missile Crises were crises created by the tensions of a nuclearized world.

Furthermore, many of the claims made about the influence of nuclear weapons and the effectiveness of nuclear deterrence rest upon characteristics that are hard to observe and almost impossible to measure. Deterrence is based upon perceptions of qualities like fear, uncertainty, and resolve — all traits that do not clearly manifest themselves prior to an event (and when deterrence “works,” no event takes place). In the absence of clear-cut ways to measure and assess these often elusive and manipulable characteristics and their effects, it becomes very hard to test the wide range of claims analysts make about the influence of nuclear weapons on grand strategy and world politics. This is not to say that we should not develop and try to test hypothesis about nuclear weapons; only that we should be careful not to overstate our claims or breezily dismiss contrary claims by thoughtful analysts.

In truth, many contemporary recommendations about U.S. nuclear policy are based on either incomplete histories or fail to appreciate the epistemological and methodological challenges behind nuclear issues. Recent scholarship has revealed that the parsimonious theories and stylized narratives we once relied on to understand U.S. nuclear weapons policy have been found wanting. For example: If nuclear weapons are such effective tools to guarantee a state’s sovereignty and security, why have so few nuclear-capable countries stopped short of deploying their own nuclear weapons? How do we explain nuclear latency or nuclear threshold states, which occupy the important “in-between” space between possessing a nuclear weapon and not? Why did the United States work with its hated adversary, the Soviet Union, to stem nuclear proliferation, even against the interests of its own allies, such as West Germany, Japan, and South Korea? Why did the United States invest and develop in massive and arguably destabilizing damage-limitation capabilities — MX, cruise missiles, the Trident D-5, Pershing II, advanced antisubmarine warfare capabilities, missile defense — soon after enshrining strategic stability in the ABM, SALT, and SALT II treaties? What influence did this investment in dramatic improvement in qualitative nuclear capabilities, at a time of quantitative nuclear balance, have on decision-makers in the Kremlin and on the outcome of the Cold War? How, if at all, does the history of the nuclear age interact with other historical strands, such as the Cold War, globalization, decolonization, and regional dynamics?

The list of unanswered questions is long and growing longer, and absent answers (or even agreement on what the right questions are), it will be difficult to achieve consensus on how nuclear weapons should be incorporated into United States grand strategy in 2017 and beyond. The good news is that the pieces in this roundtable offer an excellent start.

The field of nuclear studies is an area in which academics can make a real difference in shaping policies of obvious importance and consequence. Bridging the stovepipes between different nuclear communities, recognizing the steep challenges to understanding how nuclear weapons affect grand strategy and international relations, being willing to challenge and test well-worn conventional wisdoms, undertaking the critical historical work, and understanding the deep moral considerations that lie at the heart of these issues will all be crucial. Regardless of who became the American president in 2017, guidance on these all too important questions would be important. Under the current circumstances, it is nothing short of essential.

Francis J. Gavin is the Frank Stanton Professor in Nuclear Security Policy Studies at MIT. His writings include Nuclear Statecraft: History and Strategy in America’s Atomic Age. In January 2017, Gavin will become the inaugural director of the Henry A Kissinger Center for Global Affairs at SAIS-Johns Hopkins