Unipolarity is where there is a lack of constraints or potential rivals to the one pre-eminent power, state or ‘pole’ in the world. When a single power is overwhelmingly dominant, this is known as hegemony.

Unipolarity is a system of international relations in which one state holds the majority of power and influence. This state is known as the hegemon and is usually the most powerful and influential state in the international system. Unipolarity is a relatively rare form of international relations, as most states are either part of a multipolar system or a bipolar system. Unipolarity is often seen as a period of relative peace and stability, as the hegemon is able to exert its influence over the other states in the system.

There can be predatory hegemony, where the dominant power acts aggressively, and benign hegemony, where the dominant power acts with good intentions.

Realists, in particular neo-realists, see unipolarity and the pursuit of hegemony as the natural consequence of states seeking power and security in an anarchic system. The surest way to protect the state from threats is to become the dominant power or hegemon. Unipolarity also can have benefits for the wider international system. The dominant power can act as the ‘world’s police officer’, intervening in conflicts between other states that threaten peace and security, or preventing human-rights abuses in civil conflicts. The hegemon can be the guarantor of economic and financial stability by setting and maintaining the ground rules for economic behaviour. The terms Pax Britannicus and Pax Americana refer to the roles played by Britain and the United States, at different points in their histories, in acting as guardians of the world order.

Liberals, by contrast, argue that unipolarity does not lead to the emergence of a benign force that promotes global peace and prosperity. Rather, they fear the emergence of a predatory hegemon that desires power at all costs. Other powers come to fear the megalomania of the dominant power, leading to a security dilemma. The very process of achieving dominance creates insecurity and hostility, which inevitably leads to conflict.

US hegemony

During Ronald Reagan’s presidency (1981–89) the US achieved a commanding lead in the Cold War. The economic pressures of competing with the US undermined the Soviet economy and under Secretary-General Mikhail Gorbachev, economic and political reforms were introduced. According to the nineteenth-century French historian and political theorist Alexis de Tocqueville, the most dangerous time for a dictatorship is when it begins to reform, since that reform is likely to be too slow to please its restive population. This was certainly the case with Gorbachev’s reforms.

As the momentum behind reform gathered pace, he gave more and more power to the constituent parts of the Soviet Union, the biggest of which, by far, was Russia led by Boris Yeltsin. When communist hardliners tried to overthrow Gorbachev in 1991, it was Yeltsin who rallied the nationalist opposition, defeated the coup and then, by declaring Russia independent, led to the dismantling of the Soviet Union. The union was formally dissolved on 26 December 1991. The break-up of the Soviet Union established 15 new independent states. Russia was, of course, the biggest, but many of the others were extremely weak economically and left politically fragile from years of oppressive communism. They also faced struggles in terms of divided ethnic identities, particularly as state borders were being redefined.

What are the implications of unipolarity for global stability?

When the Soviet Union collapsed in 1991, the US achieved hegemonic status, since there was now no other state that could globally compete with it. The resulting world order that characterised the ending of the Cold War was therefore unipolar. According to Joseph Nye, ‘not since Rome has one nation loomed so large above the others’. Charles Krauthammer has referred to this period as representing the US’s ‘unipolar moment’. One aide of George W. Bush is even alleged to have gone so far as to claim that ‘we’re an empire now, and when we act, we create our own reality’. According to the hegemonic stability theory, a hegemon that is perceived by most other global players as being benign can act as a global police, and this will therefore encourage and promote global stability. The awesome and unchallengeable power of Rome provided stability in the ancient world for centuries, since no other power could challenge its authority. This long period of peace therefore became known as the Pax Romana. In the second half of the nineteenth century, the naval outreach of Great Britain also provided international stability, as British ships patrolled global sea lanes and no other power was prepared to seek to displace Great Britain as global hegemon.

At the end of the Cold War, a similar Pax Americana was also achieved. American ideals of free-market liberal democracy, as Francis Fukuyama pointed out in The End of History, were triumphant and the global popularity of the US’s economic, political and cultural identity was assured. Other powers ‘bandwagoned’ behind the US in order to secure their protection, share its ideals and avoid its wrath. The US was seen as a ‘benign leader’.

The dangers of unipolarity

Case Study America's War in Iraq

However, according to realists like Kenneth Waltz, a unipolar world can also be highly unstable. This is because the hegemonic status of one state can encourage dangerous resentment among emerging powers. Waltz argues that because states are security- maximisers, in their attempts to protect themselves, they will feel constrained by another power’s claims to global hegemony. This will be particularly dangerous and destabilising when a hegemon is declining in power and influence. Such a state of affairs has been referred to as power transition and can make international relations extremely volatile.

It has been argued that this provoked the First World War, since a rapidly growing Germany, emboldened by British failures in the Boer War, decided to challenge what had been the hegemonic status of Great Britain. According to this principle, US hegemony encouraged stability, so long as her position was unrivalled. However, the US’s failure to achieve its objectives in either Afghanistan or Iraq, followed by the collapse of the US bank Lehman Brothers, has more recently highlighted US military and economic weakness, so undermining the US’s claims to global leadership.

The attitude of emerging powers towards the existing hegemon is therefore vital. If the hegemon is resented and emerging states decide that they can achieve more by challenging it, this can create the environment for destabilising power transition. Until recently, for example, China has been prepared to accept US hegemony. However, its increasing assertiveness in its ‘near abroad’, represented by its building of reefs in the South China Sea in defiance of US-led regional condemnation, suggests that it feels able to challenge US dominance. Equally, the Russian annexation of Crimea from Ukraine in 2014 in defiance of an onslaught of Western criticism indicates that Russia may also begin to probe US weakness. The way in which President Barack Obama also refused to provide global leadership during the Arab Spring and then stood by as Russia militarily intervened in Syria on behalf of President Assad further suggests the limitations of US power.

The political philosopher Noam Chomsky argued that the possession by one state of hegemonic power is very dangerous, since a lack of constraints on what it is able to do can encourage it to act in defiance of international norms of behaviour. This can therefore create the potential for malign hegemony, in which one state becomes so powerful that it no longer takes into account the views of other states. Chomsky argues that a unipolar world can encourage a hegemon to become a ‘rogue superpower’, pursuing its own interests at the expense of international law. The way, for example, in which the US invaded Iraq in 2003 without a UN mandate demonstrates the danger of one power having such pre-eminent power that it can ignore the wishes of other states and international organs of global governance.