 Let's say that you're Russia or China, and you're looking for ways to expand your influence on the world order. Unfortunately for you, there is a major roadblock in your way, the United States' nuclear umbrella. Great power competition may be returning after a post-Cold War slumber, and adversaries may feel more confident about flexing their muscles these days, but this long-standing policy remains a bulwark of American deterrence. China and Russia are trying their hands, but their attempts to shift the world order have been obstructed by the presence of allies protected by America's nuclear arsenal, leaving them with far fewer opportunities to accomplish their objectives. So what exactly is a nuclear umbrella, and who gets to hide under its canopy? At its most basic, the United States' nuclear umbrella is a function of its system of alliances. As part of its commitment to allies, the United States has implied both the will and the means to use its nuclear weapons to protect them from aggression. The umbrella currently extends to long-time allies like NATO partners Japan, South Korea, and Australia. There is no formal binding agreement creating the nuclear umbrella, rather it is an informal understanding that the United States will defend its close allies from nuclear aggression with a retaliatory nuclear response. The nuclear umbrella therefore accomplishes two strategic goals. First, it deters other countries like Russia and an increasingly aggressive China from using their nuclear weapons or invading countries covered by the umbrella for fear of nuclear escalation. Second, it prevents nuclear proliferation as America's allies will feel less of a need to create their own nuclear arsenals to defend themselves as they can rely on the United States instead. A good example of the nuclear umbrella in practice can be found in the North Korean Security Challenge. Although North Korea has had a long-standing nuclear weapons and ballistic missile program, neither South Korea nor Japan, who are most threatened, have responded with nuclear programs of their own because of the implicit American security guarantee. Another example of the nuclear umbrella in practice would be in Eastern Europe. For historical and strategic reasons, Russia would like to reincorporate the Baltic states into its orbit. But unlike Ukraine, these countries, Estonia, Latvia and Lithuania are NATO members, with both Article 5 protection and an implicit understanding that they are under the US nuclear umbrella should Russia resort to nuclear saber-rattling. Great news for places like Japan and Lithuania, but what about the rest of the world? Some American national security experts have called for expanding the American nuclear umbrella's reach and guarantees. For example, with the potential for a nuclear armed Iran, the umbrella may need to be extended to other countries in the region, such as Saudi Arabia, lest it develop its own nuclear weapons in response and create an atomic arms race in the Middle East. Other experts disagree with nuclear umbrella expansion because they believe that it will create new risks to the United States, while increasing the reliance on nuclear weapons and atomic threats for security, and thus the incentive to create them. This is part of the reason why NATO expansion has not been universally well-received in Washington. These experts are also wary of the potentiality for nuclear escalation should the United States overexpose itself on implied commitments to use these weapons of mass destruction. Although it has been a successful policy since the start of the atomic age, the nuclear umbrella is not as simple as it sounds. To be operational, it requires not only American capability and will, but also its security partners to trust that it is committed to using it. Sounds a little paradoxical, right? Well, here's how it works. A security official in Japan perhaps best summarized the problems in that third requirement when he asked, if you were responsible for the defense of your country and you had to depend on somebody from another country, how would you feel? This message quoted in an article for the diplomat from a 2009 meeting raises the conundrum. For the US nuclear umbrella to work, cooperating countries must believe that the United States will actually defend them, and they must believe this beyond the almost ritualistic assurances that American security officials periodically make. Trust is a vital component in the otherwise informal nuclear umbrella agreement. Otherwise, these countries will have an incentive to create their own nuclear weapons despite their participation in the Nuclear Non-Proliferation Treaty. North Korea poked holes in the NPT when it withdrew from the treaty in order to develop its own nuclear weapons. Although anti-nuclear attitudes are obviously strong among American allies like Japan, long-term security concerns will tend to win out over short-term domestic political controversy. If the leaders of those countries fill the long-term vulnerability in the absence of American leadership, it's far likely that nuclear proliferation will take place. While the international order under stress from actors like China, Russia, Iran, and North Korea, the United States has signaled to its allies in recent years that it remains committed to their nuclear defense. In a 2019 speech at the Brookings Institution, David J. Trachtenberg, the then-Principal Deputy Undersecretary for Policy in the United States Department of Defense, noted that America had budgeted $25 billion toward efforts to modernize its nuclear triad, intercontinental ballistic missiles, submarine-launched ballistic missiles, and bomber-delivered nuclear weapons, plus their means of delivery. However, although the nuclear umbrella proved successful in the Cold War, some observers have questioned whether it still makes sense even in the countries that it has long extended to. Although American security officials like Trachtenberg had attempted to assure allies of its continued commitment, President Trump has raised some fears during his campaign and tenure. For example, on the campaign trail in 2016, he mulled whether it would be better if Japan and South Korea developed their own nuclear weapons to defend themselves from North Korea, instead of relying on the United States, which in his opinion, got too little out of the arrangements. Although Trump would later backtrack on these comments and made no such moves as President, his erratic nature rattled American allies and exposed this weakness in the nuclear umbrella idea. Despite the Trump years, American allies still have a high degree of trust in the United States and its commitment to their defense. After Russia's invasion of Ukraine, the polling firm Morning Consult took the opportunity and asked America's European, Japanese, and Australian allies about their trust in the U.S. nuclear umbrella, among other questions. While people in these nations viewed the nuclear weapons themselves overwhelmingly negatively, they had a high degree of trust in the United States' implicit nuclear guarantee. Only in Spain did more people, 36%, say that their country needed to create nuclear weapons rather than rely on the United States. The people in Australia, 35%, Germany, 41%, Italy, 45%, and Japan, 39%, all answered that their respective countries did not need to build nuclear weapons because their allies had them. These responses were well in excess of those who answered that their nations should build their own nuclear weapons at 28%, 28%, 20%, and 18%, respectively. Other people surveyed, said that they did not know about or have an opinion on the subject. With the unpopularity of nuclear weapons among America's allies, it seems the nuclear umbrella policy is secure for at least the intermediate future. But how exactly does the United States secure its nuclear umbrellas range? Although this is not explicitly stated, the nuclear umbrella policy also means that in practice, the United States can forward deploy its nuclear arsenal on the soil of its allied countries. Such deployment ensures that the means of delivery are closer to potential adversaries, while making it harder to eliminate American nuclear capabilities in a first strike, enhancing the overall deterrence posture. One notable hint of this practice occurred in the summer of 2022, when the United States and South Korea started talks which would eventually result in the return of the blue lightning exercises, which feature potentially nuclear capable B-52 strategic bombers flying over South Korean airspace. The last such exercise was in 2017, amid rising tensions with North Korea. Although the United States withdrew its nuclear weapons from South Korea in 1991, blue lightning and other exercises demonstrate the ongoing security operation between the two countries and the partnership's nuclear capability. The United States still deploys nuclear weapons from bases in allied territories as part of its NATO commitments. About 150 American B-61 nuclear bombs are currently on NATO soil, ready to be deployed from bases in Belgium, Germany, the Netherlands, Italy and Turkey. Although the governments of these countries have never officially confirmed their presence, it is a widely known open secret. This arrangement is part of NATO policy. Although these are American weapons and guarded by American troops, the host country would jointly need to agree to their use. Germany, Belgium and the Netherlands have at times tried to remove these weapons from their soil, but have never succeeded. There's one catch here, though. Technically, their presence violates the provisions of the MPT. Since the treaty forbids the transfer of nuclear weapons from recognized nuclear weapon states to non-nuclear weapon states. To get around this, the United States has argued that those weapons would not be effectively transferred until wartime, but as we've just seen, this practice is highly controversial. The United States also used to deploy nuclear weapons in Japan, but withdrew these in the late 1960s and early 1970s after controversy. However, Russia's invasion of Ukraine, the threat posed by a nuclear capable North Korea and a more aggressive China, prompted the late former Japanese Prime Minister Shinzo Abe to suggest that these weapons of mass destruction may need to come back to Japanese soil. In March, Abe pondered whether it would be necessary to return to a nuclear sharing arrangement for deterrence, such as that seen in NATO, stating, we should not put a taboo on discussions about the reality we face. Current Japanese PM Fumio Kishida shot the idea down, but with Russia and China unlikely to stop their attempts to increase their influence and change the world order, it is likely that such debates will continue in America's nuclear umbrella allies in the coming years. There is a final ingredient required for the nuclear umbrella to work. America's enemies must also perceive that the threat is credible, since the object of the policy is deterrence. The United States has never explicitly confirmed the nuclear umbrella as a set in stone policy and in his 2019 speech, Trachtenberg noted that it was important to keep potential adversaries like China and Russia guessing whether the United States would actually use its nuclear weapons. Because the nuclear umbrella remains more of an idea, even if a long-standing one, than formal policy doctrine or treaty, there will always remain room for interpretation among enemies and allies alike. Vladimir Putin clearly did not believe that the nuclear umbrella would be a factor before he invaded Ukraine. Ukraine, which surrendered its inherited Soviet nuclear weapons in 1994, did so on the basis that the United States, United Kingdom, and Russia would guarantee its security. Such guarantees Putin calculated turned out to be worthless. Ukraine was not a NATO member and therefore not covered under the nominal nuclear umbrella. Nevertheless, the invasion demonstrates that US security guarantees are not always iron clad, especially when they don't come in the form of a treaty. Taiwan is in much the same position that Ukraine was before the invasion. It has no formal alliance with the United States and is not covered under the nuclear umbrella. Its neighbor Japan, which China has territorial disputes with, is. Would the Chinese leadership be deterred from invading Taiwan for fear of escalation involving Japan and the nuclear umbrella? Such questions remain unanswered, but they are the most crucial plank in the nuclear umbrella. Just because something has worked in the past does not mean that it will continue to work in the future. This fact means that the United States will need to reinforce its nuclear capabilities, relationships with its participating allies, and the implication, even if informal, that they will be covered under the nuclear umbrella in the worst-case scenario of a rainy day. So what do you think? Is the American nuclear umbrella powerful enough to deter countries like Russia or China from direct and all-out attacks on other nations? What would you change, if anything, regarding this policy? Let us know in the comments, and don't forget to subscribe for more military analysis from Military Experts.