In our new era of great-power competition, it’s important to identity the competitors. But it has always been easier to speak about the great powers than to define them. Disagreement over great-power status, and especially over which power is the “greatest,” characterizes today’s system, as it did in times past. There is neither a commonly accepted definition of what constitutes a great power, nor any consensus over such basic questions as how many powers there are.

Nevertheless, we can distinguish the great powers by a set of common characteristics, which reveal that there are only four great powers that exist today—and they are not necessarily the ones you would expect.

In our new era of great-power competition, it’s important to identity the competitors. But it has always been easier to speak about the great powers than to define them. Disagreement over great-power status, and especially over which power is the “greatest,” characterizes today’s system, as it did in times past. There is neither a commonly accepted definition of what constitutes a great power, nor any consensus over such basic questions as how many powers there are.

Nevertheless, we can distinguish the great powers by a set of common characteristics, which reveal that there are only four great powers that exist today—and they are not necessarily the ones you would expect.

Great powers, first of all, have a set of behaviors in common. They always expect to shape or at least be consulted on the main global issues of the day. They make their presence felt, and their absence creates a vacuum to be filled. Often, great powers will insist on their own absolute sovereignty but admit only the qualified sovereignty of lesser powers, especially if they are nearby. In extremis, they reserve the right to change regimes that threaten or displease them but are able to deny any such right with respect to themselves.

At times, the great powers will claim to be above international law. At other times, they will make a virtue of vindicating that law or claim to defend international norms. In other words, the great powers have the power to make the rules and to break them; they are never just rule-takers. They are the orderers, not the ordered.

What enables the great powers to behave this way are their superior capabilities compared to the middling and smaller states. The first such capability is resources. Does the state in question have the military capacity to impose its will or to resist that of others? There is no entirely satisfactory way of assessing military strength, but how much the state spends on its military and how effectively is a rough measure of its defense capabilities.

Deployable nuclear weapons are also an indispensable component of great-power status today. The guaranteed ability to deliver an atomic bomb and thus to deter a nuclear attack gives a state a special position in the world. This is why the great powers take on the immense burdens of planning, researching, maintaining, storing, training, and safeguarding associated with those weapons. Not all nuclear powers are great powers, but all great powers are nuclear.

Then there is the economy. Is the state strong enough to survive the financial headwinds of geopolitical competition and to sustain a substantial military effort? Usually, economic strength is measured by GDP, which covers everything produced within a state’s borders. The alternative metric of purchasing power parity takes into account how far a sum of money goes in the domestic economy. It privileges non-Western countries with lower standards of living and production costs.

Very important—and difficult to........

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