In December 2008, shortly before being sworn in as the 44th president of the United States, Barack Obama pledged his belief that, "to ensure prosperity here at home and peace abroad", it was vital to maintain "the strongest military on the planet". Unveiling his national security team, including George Bush's defence secretary, Robert Gates, he said: "We also agree the strength of our military has to be combined with the wisdom and force of diplomacy, and that we are going to be committed to rebuilding and restrengthening alliances around the world to advance American interests and American security."

Unfortunately, many of the Obama administration's diplomatic efforts are being directed towards maintaining and garnering new access for the US military across the globe. US military officials, through their Korean proxies, have completed the eviction of resistant rice farmers from their land around Camp Humphreys, South Korea, for its expansion (including a new 18-hole golf course); they are busily making back-room deals with officials in the Northern Mariana Islands to gain the use of the Pacific islands there for bombing and training purposes; and they are scrambling to express support for a regime in Kyrgyzstan that has been implicated in the murder of its political opponents but whose Manas Airbase, used to stage US military actions in Afghanistan since 2001, Obama and the Pentagon consider crucial for the expanded war there.

The global reach of the US military today is unprecedented and unparalleled. Officially, more than 190,000 troops and 115,000 civilian employees are massed in approximately 900 military facilities in 46 countries and territories (the unofficial figure is far greater). The US military owns or rents 795,000 acres of land, with 26,000 buildings and structures, valued at $146bn (£89bn). The bases bristle with an inventory of weapons whose worth is measured in the trillions and whose killing power could wipe out all life on earth several times over.

The official figures exclude the huge build-up of troops and structures in Iraq and Afghanistan over the past decade, as well as secret or unacknowledged facilities in Israel, Kuwait, the Philippines and many other places. In just three years of the Iraq and Afghanistan wars, $2bn was spent on military construction. A single facility in Iraq, Balad Airbase, houses 30,000 troops and 10,000 contractors, and extends across 16 square miles, with an additional 12 square mile "security perimeter". From the battle zones of Afghanistan and Iraq to quiet corners of Curaçao, Korea and Britain, the US military domain consists of sprawling army bases, small listening posts, missile and artillery testing ranges and berthed aircraft carriers (moved to "trouble spots" around the world, each carrier is considered by the US navy as "four and a half acres of sovereign US territory"). While the bases are, literally speaking, barracks and weapons depots, staging areas for war-making and ship repairs, complete with golf courses and basketball courts, they are also political claims, spoils of war, arms sale showrooms and toxic industrial sites. In addition to the cultural imperialism and episodes of rape, murder, looting and land seizure that have always accompanied foreign armies, local communities are now subjected to the ear-splitting noise of jets on exercise, to the risk of helicopters and warplanes crashing into residential areas, and to exposure to the toxic materials that the military uses in its daily operations.

The global expansion of US bases - and with it the rise of the US as a world superpower - is a legacy of the Second World War. In 1938, the US had 14 military bases outside its continental borders. Seven years later, it had 30,000 installations in roughly 100 countries. While this number was projected to shrink to 2,000 by 1948 (following pressure from other nations to return bases in their own territory or colonies, and pressure at home to demobilise the 12 million-man military), the US continued to pursue access rights to land and air space around the world. It established security alliances with multiple states within Europe (Nato), the Middle East and south Asia (Cento) and south-east Asia (Seato), as well as bilateral agreements with Japan, Taiwan, South Korea, Australia and New Zealand. Status of Forces Agreements (Sofas) were crafted in each country to specify what the military could do, and usually gave US soldiers broad immunity from prosecution for crimes committed and environmental damage caused. These agreements and subsequent base operations have mostly been shrouded in secrecy, helped by the National Security Act of 1947. New US bases were built in remarkable numbers in West Germany, Italy, Britain and Japan, with the defeated Axis powers hosting the most significant numbers (at one point, Japan was peppered with 3,800 US installations).

As battles become bases, so bases become battles; the sites in east Asia acquired during the Spanish-American war in 1898 and during the Second World War - such as Guam, Thailand and the Philippines - became the primary bases from which the US waged war on Vietnam. The number of raids over north and south Vietnam required tons of bombs unloaded at the naval station in Guam. The morale of ground troops based in Vietnam, as fragile as it was to become through the latter part of the 1960s, depended on R&R (rest and recreation) at bases outside the country, which allowed them to leave the war zone and yet be shipped back quickly and inexpensively for further fighting. The war also depended on the heroin the CIA was able to ship in to the troops on the battlefield in Vietnam from its secret bases in Laos. By 1967, the number of US bases had returned to 1947 levels.

Technological changes in warfare have had important effects on the configuration of US bases. Long-range missiles and the development of ships that can make much longer runs without resupply have altered the need for a line of bases to move forces forward into combat zones, as

has the aerial refuelling of military jets. An arms airlift from the US to the British in the Middle East in 1941-42, for example, required a long hopscotch of bases, from Florida to Cuba, Puerto Rico, Barbados, Trinidad, British Guiana, north-east Brazil, Fernando de Noronha, Takoradi (now in Ghana), Lagos, Kano (now in Nigeria) and Khartoum, before finally making delivery in Egypt. In the early 1970s, US aircraft could make the same delivery with one stop in the Azores, and today can do so non-stop.

On the other hand, the pouring of money into military R&D (the Pentagon has spent more than $85bn in 2009), and the corporate profits to be made in the development and deployment of the resulting technologies, have been significant factors in the ever larger numbers of technical facilities on foreign soil. These include such things as missile early-warning radar, signals intelligence, satellite control and space-tracking telescopes. The will to gain military control of space, as well as gather intelligence, has led to the establishment of numerous new military bases in violation of arms-control agreements such as the 1967 Outer Space Treaty. In Colombia and Peru, and in secret and mobile locations elsewhere in Latin America, radar stations are primarily used for anti-trafficking operations.

Since 2000, with the election of George W Bush and the ascendancy to power of a group of men who believed in a more aggressive and unilateral use of military power (some of whom stood to profit handsomely from the increased military budget that would require), US imperial ambition has grown. Following the declaration of a war on terror and of the right to pre-emptive war, the number of countries into which the US inserted and based troops radically expanded. The Pentagon put into action a plan for a network of "deployment" or "forward operating" bases to increase the reach of current and future forces. The Pentagon-aligned, neoconservative think tank the Project for the New American Century stressed that "while the unresolved conflict with Iraq provides the immediate justification, the need for a substantial American force presence in the Gulf transcends the issue of the regime of ­Saddam Hussein".

The new bases are designed to operate not defensively against particular threats but as offensive, expeditionary platforms from which military capabilities can be projected quickly, anywhere. The Global Defence Posture Review of 2004 announced these changes, focusing not just on reorienting the footprint of US bases away from cold war locations, but on remaking legal arrangements that support expanded ­military activities with other allied countries and prepositioning equipment in those countries. As a recent army strategic document notes, "Military personnel can be transported to, and fall in on, prepositioned equipment significantly more quickly than the equivalent unit could be transported to the theatre, and prepositioning equipment overseas is generally less politically difficult than stationing US military personnel."

Terms such as facility, outpost or station are used for smaller bases to suggest a less permanent presence. The US department of defence currently distinguishes between three types of military facility. "Main operating bases" are those with permanent personnel, strong infrastructure, and often family housing, such as Kadena Airbase in Japan and Ramstein Airbase in Germany. "Forward operating sites" are "expandable warm facilit[ies] maintained with a limited US military support presence and possibly prepositioned equipment", such as Incirlik Airbase in Turkey and Soto Cano Airbase in Honduras. Finally, "co-operative security locations" are sites with few or no permanent US personnel, maintained by contractors or the host nation for occasional use by the US military, and often referred to as "lily pads". These are cropping up around the world, especially throughout Africa, a recent example being in Dakar, Senegal.

Moreover, these bases are the anchor - and merely the most visible aspect - of the US military's presence overseas. Every year, US forces train 100,000 soldiers in 180 countries, the presumption being that beefed-up local militaries will help to pursue US interests in local conflicts and save the US money, casualties and bad publicity when human rights abuses occur (the blowback effect of such activities has been made clear by the strength of the Taliban since 9/11). The US military presence also involves jungle, urban, desert, maritime and polar training exercises across wide swathes of landscape, which have become the pretext for substantial and permanent positioning of troops. In recent years, the US has run around 20 exercises annually on Philippine soil, which have resulted in a near-continuous presence of US soldiers in a country whose people ejected US bases in 1992 and whose constitution forbids foreign troops to be based on its territory. Finally, US personnel work every day to shape local legal codes to facilitate US access: they have lobbied, for example, to change the Philippine and Japanese constitutions to allow, respectively, foreign troop basing and a more-than-defensive military.

Asked why the US has a vast network of military bases around the world, Pentagon officials give both utilitarian and humanitarian arguments. Utilitarian arguments include the claim that bases provide security for the US by deterring attack from hostile countries and preventing or remedying unrest or military challenges; that bases serve the national economic interests of the US, ensuring access to markets and commodities needed to maintain US standards of living; and that bases are symbolic markers of US power and credibility - and so the more the better. Humanitarian arguments present bases as altruistic gifts to other nations, helping to liberate or democratise them, or offering aid relief. None of these humanitarian arguments deals with the problem that many of the bases were taken during wartime and "given" to the US by another of the war's victors.

Critics of US foreign policy have dissected and dismantled the arguments made for maintaining a global system of military basing. They have shown that the bases have often failed in their own terms: despite the Pentagon's claims that they provide security to the regions they occupy, most of the world's people feel anything but reassured by their presence. Instead of providing more safety for the US or its allies, they have ­often provoked attacks, and have made the communities around bases key targets of other nations' missiles. On the island of Belau in the Pacific, the site of sharp resistance to US attempts to instal a submarine base and jungle training centre, people describe their experience of military basing in the Second World War: "When soldiers come, war comes." On Guam, a joke among locals is that few people except for nuclear strategists in the Kremlin know where their island is.

As for the argument that bases serve the national economic interest of the US, the weapons, personnel and fossil fuels involved cost billions of dollars, most coming from US taxpayers. While bases have clearly been concentrated in countries with key strategic resources, particularly along the routes of oil and gas pipelines in central Asia, the Middle East and, increasingly, Africa, from which one-quarter of US oil imports are expected by 2015, the profits have gone first of all to the corporations that build and service them, such as Halliburton. The myth that bases are an altruistic form of "foreign aid" for locals is exploded by the substantial costs involved for host economies and polities. The immediate negative effects include levels of pollution, noise, crime and lost productive land that cannot be offset by soldiers' local spending or employment of local people. Other putative gains tend to benefit only local elites and further militarise the host nations: elaborate bilateral negotiations swap weapons, cash and trade privileges for overflight and land-use rights. Less explicitly, rice imports, immigration rights to the US or overlooking human rights abuses have been the currency of exchange.

The environmental, political, and economic impact of these bases is enormous. The social problems that accompany bases, including soldiers' violence against women and car crashes, have to be handled by local communities without compensation from the US. Some communities pay the highest price: their farmland taken for bases, their children neurologically damaged by military jet fuel in their water supplies, their neighbours imprisoned, tortured and disappeared by the autocratic regimes that survive on US military and political support given as a form of tacit rent for the bases. The US military has repeatedly interfered in the domestic affairs of nations in which it has or desires military access, operating to influence votes and undermine or change local laws that stand in the way.

Social movements have proliferated around the world in response to the empire of US bases, ever since its inception. The attempt to take the Philippines from Spain in 1898 led to a drawn-out guerrilla war for independence that required 126,000 US occupation troops to stifle. Between 1947 and 1990, the US military was asked to leave France, Yugoslavia, Iran, Ethiopia, Libya, Sudan, Saudi Arabia, Tunisia, Algeria, Vietnam, Indonesia, Peru, Mexico and Venezuela. Popular and political objection to the bases in Spain, the Philippines, Greece and Turkey in the 1980s gave those governments the grounds to negotiate ­significantly more compensation from the US. Portugal threatened to evict the US from important bases in the Azores unless it ceased its support for independence for its African colonies.

Since 1990, the US has been sent packing, most significantly, from the Philippines, Panama, Saudi Arabia, Vieques and Uzbekistan. Of its own accord, for varying reasons, it decided to leave countries from Ghana to Fiji. Persuading the US to clean up after itself - including, in Panama, more than 100,000 rounds of unexploded ordnance - is a further struggle. As in the case of the US navy's removal from Vieques in 2003, arguments about the environmental and health damage of the military's activities remain the centrepiece of resistance to bases.

Many are also concerned by other countries' overseas bases - primarily European, Russian and Chinese - and by the activities of their own militaries, but the far greater number of US bases and their weaponry has understandably been the focus. The sense that US bases represent a major injustice to the host community and nation is very strong in countries where US bases have the longest standing and are most ubiquitous. In Okinawa, polls show that 70 to 80 per cent of the island's people want the bases, or at least the marines, to leave. In 1995, the abduction and rape of a 12-year-old Okinawan girl by two US marines and one US sailor led to demands for the removal of all US bases in Japan. One family in Okinawa has built a large peace museum right up against the edge of the Futenma Airbase, with a stairway to the roof that allows busloads of schoolchildren and other visitors to view the sprawling base after looking at art depicting the horrors of war.

In Korea, the great majority of the population feels that a reduction in US presence would increase national security; in recent years, several violent deaths at the hands of US soldiers triggered vast candlelight vigils and protests across the country. And the original inhabitants of Diego Garcia, evicted from their homes between 1967 and 1973 by the British on behalf of the US for a naval base, have organised a concerted campaign for the right to return, bringing legal suit against the British government, a story told in David Vine's recent book Island of Shame. There is also resistance to the US expansion plans into new areas. In 2007, a number of African nations baulked at US attempts to secure access to sites for military bases. In eastern Europe, despite well-funded campaigns to convince Poles and Czechs of the value of US bases and much sentiment in favour of accepting them in pursuit of closer ties with Nato and the EU, and promised economic benefits, vigorous pro­tests have included hunger strikes and led the Czech government, in March, to reverse its plan to allow a US military radar base to be built in the country.

The US has responded to action against bases with a renewed emphasis on "force protection", in some cases enforcing curfews on soldiers, and cutting back on events that bring local people on to base property. The department of defence has also engaged in the time-honoured practice of renaming: clusters of soldiers, buildings and equipment have become "defence staging posts" or "forward operating locations" rather than military bases. Regulating documents become "visiting forces agreements", not "status of forces agreements", or remain entirely secret. While major reorganisation of bases is under way for a host of reasons, including a desire to create a more mobile force with greater access to the Middle East, eastern Europe and central Asia, the motives also include an attempt to prevent political momentum of the sort that ended US use of the Vieques and Philippine bases.

The attempt to gain permanent basing in Iraq foundered in 2008 on the objections of forces in both Iraq and the US. Obama, in his Cairo speech in June, may have insisted that "we pursue no bases" in either Iraq or Afghanistan, but there has been no sign of any significant dismantling of bases there, or of scaling back the US military presence in the rest of the world. The US secretary of state, Hillary Clinton, recently visited Japan to ensure that it follows through on promises to provide the US with a new airfield on Okinawa and billions of dollars to build new housing and other facilities for 8,000 marines relocating to Guam. She ignored the invitation of island activists to come and see the damage left by previous decades of US base activities. The myriad land-grabs and hundreds of billions of dollars spent to quarter troops around the world persist far beyond Iraq and Afghanistan, and too far from the headlines.

Catherine Lutz is a professor at the Watson Institute for International Studies at Brown University and editor of "The Bases of Empire: the Global Struggle against US Military Posts" (Pluto Press, £17.99)