In 1492, Columbus sailed west. In 1991, the Soviet Union collapsed. These two events bracketed the European age. Once, Mayans lived unaware that there were Mongols, who were unaware there were Zulus. From the 15th century onwards, European powers collectively overwhelmed the world, creating the first truly global geopolitical system in human history, to the point where the fate of Australian Aborigines was determined by British policy in Ireland and the price of bread in France turned on the weather in Minnesota.

Europe simultaneously waged a 500-year-long civil war of increasing savagery, until the continent tore itself apart in the 20th century and lost its hold on the world. After the collapse of the Soviet Union, there was no longer a single European nation that could be considered a global power of the first rank.

Another unprecedented event took place a decade or so earlier. For 500 years, whoever controlled the North Atlantic controlled Europe's access to the world and, with it, global trade. By 1980, the geography of trade had shifted, so that the Atlantic and Pacific were equally important, and any power that had direct access to both oceans had profound advantages. North America became the pivot of the global system, and whatever power dominated North America became its centre of gravity. That power is, of course, the United States.

It is geography combined with the ability to exploit it that matters. The US is secure from attack on land or sea. It is vulnerable to terrorist attack but, outside of a nuclear exchange, faces no existential threat in the sense that Britain and France did in 1940-41, or Germany and Japan did in 1944-45. Part of its advantage is that, alone among the combatants, the US actually profited from the Second World War, emerging with a thoroughly modernised industrial base. But this itself can be traced to the country's core geography. The fertility of the land between the Appa­lachians and the Rocky Mountains, and the configuration of the country's river system, drove an economic system in the 19th century that helped fund an economy which today constitutes between 25 and 30 per cent of global economic activity, depending on how you value the dollar.

Just as important, perhaps, is that while the population density of Japan is about 365 people per square kilometre and that of most European states between 100 and 300 per square kilometre, the US population density, excluding Alaska, is about 34 people per square kilometre. The US has room to grow and it manages immigration well. Its population is not expected to decline. It is the pre-eminent power not because of the morality of the regime, the virtue of its people or the esteem in which it is held, but because of Europe's failures and changes in global trade patterns.

This is a geopolitical reading of history. Geo­politics argues that it is geography which defines power, and that military, economic and political power are different parts of a single system. Geopolitics tends not to take policies or politicians very seriously, seeing them as trapped in reality. The finest statesman ruling Iceland will not dominate the world; the stupidest ruling ancient Rome could not undermine its power.

Economists talk about an invisible hand - a concept, if not a term, they have borrowed from Machiavelli. Geopolitics applies the concept of the invisible hand to the behaviour of nations and other international actors. Geopolitics and economics both hold that the players are rational and will pursue their self-interest, if not flawlessly, then at least not randomly.

Think of a chess game. On the surface, it appears that each player has 20 potential opening moves. In fact, there are many fewer, because most of these moves are so bad that they would quickly lead to defeat. The better you are at chess, the more clearly you see your options, and the fewer moves you regard as being available: the better the player, the more predictable the move. The grandmaster plays with absolute predictable precision - until that one brilliant, unexpected stroke.

Geopolitics assumes two things: first, that human beings organise themselves into units larger than families and that they have a natural loyalty to the things they were born into, the people and the places; second, that the character of a nation is determined to a great extent by geography, as is the relationship between nations. We use the term "geography" broadly. It includes the physical characteristics of a location, but it goes beyond that to look at the effects of a place on individuals and communities. These are the foundation of geopolitical forecasting.

Opinion and reputation have little to do with national power. Whether the US president is loathed or admired is of some minor immediate import, but the fundamentals of power are overarching. Nor do passing events have much to do with national power, no matter how significant they appear at that moment. The recent financial crisis mattered, but it did not change the basic geometry of international power. The concept of American decline is casually tossed about, but for America to decline, some other power must surpass it. There are no candidates.

Consider China, most often mentioned as the challenger to the US. Han China is surrounded by four buffer states, Manchuria, Inner Mongolia, Xinjiang and Tibet. Without these buffers, the borders of China move inward and China becomes vulnerable. With these four buffers in place, China is secure - but as a landlocked island, bounded by mountainous jungle, the Himalayas, the steppes of central Asia and the Siberian wasteland. China is blocked in all directions but the sea.

The vast majority of China's population lives within a thousand miles of the Pacific coast. Beyond this line, water supply will not support large populations. Most industrial development has taken place within a hundred miles of the coast. Consider the following numbers, culled from official Chinese statistics. About 65 million Chinese people live in households with more than $20,000 a year in income. Around 165 million make between $2,000 and $20,000 a year. Most of these live within 100 miles of the coast. About 400 million Chinese have household ­incomes between $1,000 and $2,000 a year, while about 670 million have household incomes of less than $1,000 a year. China is a land of extra­ordinary poverty. Mao made the Long March to raise an army of desperate peasants to rectify this sort of extreme imbalance. The imbalance is there again, a volcano beneath the current regime.

China would have to triple the size of its economy - and the US would have to stand still - if China were to pull even with the US in GDP. Militarily, China is impotent. Its army is a domestic security force, its ability to project power blocked by natural barriers. Its navy exists mostly on paper and could not possibly pose a serious threat to the US. Casual assertions of China surpassing the US geopolitically ignore fundamental, overwhelming realities. China could conceivably overcome its problems, but it would require most of the century to overcome problems of this magnitude.

Europe, if it ever coalesced into a unified economic and military power, could certainly challenge the US. However, as we have seen during the recent financial crisis, nationalism continues to divide the continent, even if exhaustion has made that nationalism less virulent. The idea of Europe becoming a multinational state with a truly integrated economic decision-making system - and with a global military force under joint command - is as distant a dream as that of China becoming a global power.

This is not an Americentric view of the world. The world is Americentric. The US marshals the economic resources of North America, controls the world's oceans and space, projects force where it wishes - wisely or not. The US is to the world what Britain once was to Europe. Both nations depended on control of the sea to secure their interests. Both nations understood that the best way to retain control of the sea was to prevent other nations from building navies. Both understood that the best way to do that was to maintain a balance of power in which potential challengers spent their resources fighting each other on land, rather than building fleets that could challenge their control of the sea.

The US is doing this globally. Its primary goal is always to prevent the emergence of a single power that can dominate Eurasia and the European peninsula. With the Soviet Union's collapse, China's limits and the EU's divisions, there is currently no threat of this. So the US has moved to a secondary goal, which is to block the emergence of any regional hegemon that could, in the long term, grow into something more dangerous. The US does what it can to disrupt the re-emergence of Russian national power while building relations with bordering countries such as Poland and Turkey. It encourages unrest in China's border regions, using the ideology of human rights as justification. It conducts direct or surrogate wars on a seemingly random basis, from Somalia to Serbia, from Iraq to Afghanistan.

Many of these wars appear to go badly. However, success is measured not by the pacification of a country, but by its disruption. To the extent that the Eurasian land mass is disrupted, to the extent that there is perpetual unrest and disunion from the Atlantic to the Pacific, the US has carried out its mission. Iraq is paradigmatic. The US intervention resulted in a civil war. What appeared to be a failure was, in fact, a satisfactory outcome. Subjectively, we would think George W Bush and his critics were unaware of this. But that is the point of geopolitics. The imperatives generate ideologies (a democratic Iraq) and misconceptions (weapons of mass destruction). These, however, are shadows on the wall. It is the geopolitical imperatives, not the rhetoric, that must be understood in order to make sense of what is going on.

Thus, the question is how these geopolitical and strategic realities shape the rest of the century. Eurasia, broadly understood, is being hollowed out. China is far weaker than it appears and is threatened with internal instability. The Europeans are divided by old national patterns that prevent them from moving in a uniform direction. Russia is using the window of opportunity presented by the US absorption in disrupting the Islamic world to reclaim its sphere of influence in the former Soviet Union, but its underlying weakness will reassert itself over the next generation.

New powers will emerge. In the 19th century, Germany, Italy and Japan began to emerge as great powers, while in the 20th century global powers such as Britain and France declined to secondary status. Each century, a new constellation of powers forms that might strike observers at the beginning of the century as unthinkable. Let us therefore think about the unthinkable.

The United States conducts an incautious foreign policy. The relative power of the US is such that it has a margin of error far beyond that of the countries it confronts. It also has a strategic disruptive imperative, based on geopolitical interests. This will make the planet an uncomfortable place, particular for rising powers.

There is another dimension built into US foreign policy - using subordinate regional powers as surrogates, exchanging their willingness to incur risks from a major power opposed to the US for substantial benefits. These range from strategic guarantees and support against smaller neighbours to trade advantages and technology transfers. The recovery of West Germany and Japan during the cold war are classic examples of this. There are three nations that are already major or emerging regional powers that will be important to the US in dealing with Russia in the next decade or so: Japan, Turkey and Poland.

Japan is already a great power. It is the world's second-largest economy, with a far more stable distribution of income and social structure than China. It has east Asia's largest navy - one that China would like to have - and an army larger than Britain's (since the Second World War, both Japan's "army" and "navy" have officially been non-aggressive "self-defence forces"). It has not been a dynamic country, militarily or economically, but dynamism comes and goes. It is the fundamentals of national power, relative to other countries, that matter in the long run.

Turkey is now the world's 17th-largest economy and the largest Islamic economy. Its military is the most capable in the region and is also probably the strongest in Europe, apart from the British armed forces. Its influence is already felt in the Caucasus, the Balkans, central Asia and the Arab world. Most important, it is historically the leader in the Muslim world, and its bridge to the rest of the world. Over the centuries, when the Muslim world has been united, this has happened under Turkish power; the past century has been the aberration. If Russia weakens, Turkey emerges as the dominant power in the region, including the eastern Mediterranean; Turkey is an established naval power. It has also been historically pragmatic in its foreign policies.

Poland has the 18th-largest economy in the world, the largest among the former Soviet satellites and the eighth-largest in Europe. It is a vital strategic asset for the US. In the emerging competition between the US and Russia, Poland represents the geographical frontier between Europe and Russia and the geographical foundation of any attempt to defend the Baltics. Given the US strategic imperative to block Eurasian hegemons and Europe's unease with the US, the US-Polish relationship becomes critical. In 2008 the US signed a deal with Poland to instal missiles in the Baltic Sea as part of Washington's European missile defence shield, ostensibly to protect against "rogue states". The shield is not about Iran, but about Poland as a US ally - from the American and the Russian points of view.

To gauge what it means for a country to be a strategic asset of a global power, consider the case of South Korea. Any suggestion in 1950 that it would become a major industrial power by the end of the century would have been greeted with disbelief. Yet that is what Korea became. Like Israel, South Korea formed a strategic relationship with the US that was transformative. And both South Korea and Israel started with a much weaker base in 1950 than Poland has today.

Russia cannot survive its economic and demographic problems indefinitely. China must face its endemic social problems. So, imagine an unstable, fragmented Eurasia. On its rim are three powers - Japan to the east, Turkey to the south and Poland to the west. Each will have been a US protégé during the Russian interregnum, but by mid-century the US tendency to turn on allies and make allies of former enemies will be in play, not out of caprice but out of geopolitical necessity.

Two of the three major powers will be maritime powers. By far the most important will be Japan, whose dependence on the importation of virtually all raw materials forces it to secure its sea lanes. Turkey will have a lesser but very real interest in being a naval power in the eastern Mediterranean, and as its power in the Muslim world rises it will develop a relationship with Egypt that will jeopardise the Suez Canal and, beyond it, the Arabian Sea. Poland, locked between Russia and Germany, and far more under US control than the other two, will be a land power.

US strategy considers any great power with significant maritime capabilities a threat; it will have solved one problem - the Russian problem - by generating another. Imagining a Japanese-Turkish alliance is strange but no stranger than a Japanese-German alliance in 1939. Both countries will be under tremendous pressure from the established power. Both will have an interest in overthrowing the global regime the US has imposed. The risk of not acting will be greater than the risk of acting. That is the basis of war.

Imagining the war requires that we extrapolate technology. For the US, space is already the enabler of its military machine. Communications, navigation and intelligence are already space-based. Any great power challenging the US must destroy US space-based assets. That means that, by the middle of the century, the US will have created substantial defences for those assets. But if the US can be rendered deaf, dumb and blind, a coalition of Turkey and Japan could force the US to make strategic concessions.

War depends on surprise, and this surprise will have to focus on the destruction of US space forces. If this sounds preposterous, then imagine how the thought of a thousand bomber raids in the Second World War would have sounded in 1900. The distance travelled technologically between 1900 and 1945 was much greater than the one I am suggesting by 2050. There are no breakthroughs required here, only developments of what already exists.

It is difficult to imagine an American defeat in this war, although not major setbacks. The sheer weight of power that the US and its Polish ally can throw against the Japanese and Turks will be overwhelming. The enemy will be trying to deny the US what it already has, space power, without being able to replace it. The US will win in a war where the stakes will be the world, but the cost will be much less than the bloody slaughters of Europe's world wars. Space does not contain millions of soldiers in trenches. War becomes more humane.

The ultimate prize is North America. Until the middle of the 19th century, there were two contenders for domination - Washington and Mexico City. After the American conquest of northern Mexico in the 1840s, Washington dominated North America and Mexico City ruled a weak and divided country. It remained this way for 150 years. It will not remain this way for another hundred. Today, Mexico is the world's 13th-largest economy. It is unstable due to its drug wars, but it is difficult to imagine those wars continuing for the rest of the century. The heirs of today's gangsters will be on the board of art museums soon enough.

Mexico has become a nation of more than 100 million people with a trillion-dollar economy. When you look at a map of the borderland between the United States and Mexico, you see a huge flow of drug money to the south and the flow of population northward. Many areas of northern Mexico that the US seized are now being repopulated by Mexicans moving northward - US citizens, or legal aliens, or illegal aliens. The political border and the cultural border are diverging.

Until after the middle of the century, the US will not respond. It will have concerns elsewhere and demographic shifts in the US will place a premium on encouraging Mexican migration northward. It will be after the mid-century systemic war that the new reality will emerge. Mexico will be a prosperous, powerful nation with a substantial part of its population living in the American south-west, in territory that Mexicans regard as their own.

The 500 years of European domination of the international system did not guarantee who would be the dominant European power. Nor is there any guarantee who will be the dominant power in North America. One can imagine scenarios in which the US fragments, in which Mexico becomes an equal power, or in which the US retains primacy for centuries, or an outside power makes a play. North America is the prize.

In due course, the geopolitical order will shift again, and the American epoch will end. Perhaps even sooner, the power of the US will wane. But not yet, and not in this century.

George Friedman is the founder of the private intelligence corporation Stratfor. His book The Next 100 Years is published by Allison & Busby (£14.99)

http://www.stratfor.com/