The empire is dead. Long live the empire.
This largest form of human social organization imploded spectacularly over the course of the past century with the collapse of European colonial empires and the Soviet Union. Still, the worm may turn yet again. While micro-states or voluntary networks of powerful transnational organizations might eventually end up being the polities of choice in our new century, we're still not free from the specter of a massively successful conquest state. Earlier waves of expansionist societies trod across the bones of their predecessors, even as they drew inspiration from those same long-expired empires. In our present age of jet aircraft, globe-spanning communications technology, and a vast, logistically networked fleet of merchant marine vessels, a next generation empire could conceivably dominate an entire hemisphere or even the planet. That's to say nothing of the potentials for one-sided global power projection in a dawning space age.
Rule From Imbalance
The rise of a global or hemispheric empire would likely require a massive technological imbalance: A single side with military tools and civilian infrastructure that vastly outclasses its opponents. Or perhaps with cyberwarfare entities capable of occupying the banking, communications, and commercial systems of subjugated client-states, while defending the online metropolitan homeland from harm in kind.
More importantly, it would require a willingness to periodically use lopsided kinetic or cybernetic force against conquered civilian populations - or perhaps against their infrastructure - whenever they grow restless under the yoke. Past empires were ultimately maintained with waves of brutal repression. The history of large conquest polities, from the Roman to the Spanish to the early Chinese and the Russian empires, all involved long bouts of ethnic cleansing, the enslavement of beaten populations (sometimes in a very literal sense to help pay for the cost of conquest), the killing of intellectuals, mass mutilation (thousands of slit noses in the case of restive Cossacks incorporated into the Russian Empire), the destruction or suppression of the cultural works of the conquered, as well as public torture and executions on a nauseating scale.
At present, the return of empires isn't the most likely of scenarios in international affairs. Technological power is too diffuse, and as technology continues to put greater amounts of lethal force into individual hands, the ability of a conquered population to create grief on a mass scale for the occupiers makes empires less than economically feasible. We also have trade agreements and international organizations like the WTO, which have helped to create global markets that allow the great powers to buy the kind of resources once commonly referred to as being strategic, without any need to go to war with one another. That's a testament to the success of these pacts and soft power economic organizations, which were created during the 1940s and 50s specifically to help prevent a third world war.
Additionally, there exists a never before seen ability to communicate news of atrocities in graphic form. An aspiring imperial power would likely face increasing resistance abroad and possibly at home as photographs and video clips emerged online. The world is smaller place these days, and killings that would have once remained only dry rumors or bloodless written reports can end up confronting people with scenes of horror on the screens of computers at home and at work.
Rule From Above and Within
Past empires typically had one or two advantages over those they forced to submit. In the case of classical Eurasian empires, it was often the ability to marshal large bodies of well-organized soldiers. Enough to beat and, just as importantly, successfully occupy the nations and chiefdoms they went to war with. With later European empires, it was typically a lopsided advantage in industrial arms technology, which let small expeditionary forces defeat Non-European enemies on their home ground during key engagements.
What would be the near future equivalents?
Space: If one nation should come to dominate low orbit in the next few decades it would be in a position to rapidly project punitive force around the globe, and control or block satellite communications. It could also deny or at least restrict access to the moon, resource-rich Near-Earth Asteroids, and the high ground of geostationary orbit.
Cyberwarfare: Dominance in cyberspace might be even more decisive. A nation that could occupy the critical online spaces of other countries' financial, infrastructure, and commercial networks with malware entities might have unparalleled power. More so than any previous empire with the mere ability to put boots (or sandals as it were) on each important street corner and in major public buildings. Imagine instead a centuries-long occupation in which the conquers are equipped with surveillance powers over beaten populations that the Soviet Union could have only dreamed of. That, and the ability to damage markets, cut power, and deny basic civil infrastructure services by remote control.
The recent surge in online eavesdropping competency and software weapons in the hands of nation-states makes this one worth thinking about.
Autonomous weapons and augmented troops: Another possibility is that several leading post-industrial nations could chose to forsake weapons automation and human enhancement technologies for ethical reasons, leaving one state free to pull ahead decisively in the sphere of military affairs. This might lead to the necessary out-of-kilter military balance required for the rise of an empire, and it could also give an imperial state wholly new abilities with which to enforce an occupation. Sufficiently advanced bio- and nanotechnology capable of interfacing with the brain could occupy the minds of its victims, rewriting their emotions along with their perception of the world and their place in it.
Rule and Division
Empires were built as much on strategy as technology and fighting organization. A near-future empire might follow a one of several roads to power.
Among these are
Hit 'em in Their Failed States: A rising empire could first occupy failed or near-failed states, and then ruthlessly oppress the majority of the population while exalting an ethnic or religious minority to ruling class client status. Or, conversely, suppress violent and disruptive elements that make life in a failed state so miserable, causing the majority to tolerate the occupation for the relative peace and prosperity it brings. This could be a tricky strategy to pursue, however, in areas where power and violence go hand-in-glove with firearms-backed chieftainships. An attack on the violent elements means wading hip if not neck deep into a morass of revenge killings, feuds, and extended families, all of which are tied into the local power structures.
Even then, a successful occupation of several failed states, however bloody initially, could gain a conquest society access to resources in a future age of global economic uncertainty, and also provide a network of launchpads for future conquests.
Throttling globalism: We live in a world of 10,000 mile supply chains and just in time manufacturing and in-store inventories. Terrorist attacks on September 11th, 2001, and an Icelandic volcano that got uppity for a couple of weeks a few years back, were both sufficient to give that physical network of transported goods and materials a case of the shivers. One route to empire could be for an aspirant power to take control of the physical choke points critical to our globalized economy. This might include occupying key points for international shipping (probably some of the same narrows associated with piracy during the past ten years), nexuses of international fiber-optic lines, and establishing air dominance over major air traffic corridors, which are based on a combination of the shortest international routes between cities and the curve of the Earth.
After all, why conquer nations directly when you can starve them of the external flows of data and materials they've become dependent on?
Next up: Global democracy and bio-nano collectives