After 1945 the United States chose to keep a large military, station much of it abroad and bind itself by treaty to defend other countries. That choice still shapes the world today, from NATO's 32 members to the warships that patrol busy sea lanes and the GPS signal in your phone.4 It has also cost trillions of dollars, drawn America into long wars and caused lasting arguments about who should pay.21 This chapter looks at how the system works now, what it costs, where US forces are based and what critics and defenders say about it.
An order built on alliances
The security system the United States built after the Second World War rests on a simple idea: it is better to deter a war than to fight one. Instead of waiting for trouble, the US tied other countries to itself by treaty and kept forces close to likely flashpoints.
The cornerstone is NATO. Twelve countries from Europe and North America signed the North Atlantic Treaty in Washington on 4 April 1949.4 Its core promise, Article 5, says an armed attack against one member "shall be considered an attack against them all". It has been invoked only once, and not to defend Europe: after the 11 September 2001 attacks on the United States. Allies then sent radar surveillance aircraft to patrol American skies and warships to patrol the Mediterranean.5
NATO now has 32 members. Finland joined in April 2023 and Sweden in March 2024, both ending long traditions of military non-alignment after Russia's full-scale invasion of Ukraine in 2022.4 The alliance's first military commander was an American, General Dwight D. Eisenhower, appointed in December 1950.7
In Asia and the Pacific there is no single alliance like NATO. Instead, the US has separate treaties with five regional allies: Japan, South Korea, Australia, the Philippines and Thailand. It also works closely with partners that are not treaty allies, including India, Indonesia, Singapore, Vietnam and Taiwan.8
Behind these treaties sits nuclear deterrence. The US promises to defend its allies with all its forces, including nuclear weapons if necessary. This is called extended deterrence, or the "nuclear umbrella", and one aim is that allies such as Japan, South Korea or Germany feel no need to build bombs of their own.
The Federation of American Scientists estimates that the US had about 3,700 warheads in its military stockpile at the start of 2026, including about 100 non-strategic bombs deployed at bases in Europe. Russia had an estimated 4,400. Nine countries had roughly 12,187 warheads between them, and the US and Russia held about 86% of the total.16
Arms control between them has now lapsed. The New START treaty, which capped both countries' deployed strategic arsenals, expired on 5 February 2026. For the first time in more than half a century, there are no binding limits on the two largest nuclear arsenals.17
Who spends the most
World military spending reached a record $2,887 billion in 2025, rising for the 11th year in a row. The US spent $954 billion, about a third of the total and 2.8 times as much as China.1
The biggest military spenders, 2025
Billions of US dollars, current prices
US spending actually fell by 7.5% in 2025, mainly because no new military aid for Ukraine was approved that year. Fast growth elsewhere, especially in Europe, still pushed the world total up.1
Spending is highly concentrated: the top five countries accounted for 58% of the world total. Some smaller budgets are huge relative to the economy. Russia spent an estimated 7.5% of its GDP on the military in 2025, and Ukraine, defending itself against Russia's invasion, about 40%.1
Figures for China, Russia, Ukraine and Saudi Arabia are SIPRI estimates, because those governments publish incomplete data. Dollar comparisons also ignore differences in wages and prices between countries, so the same sum can buy more in some places than in others.
What the money buys
Measured against the size of its economy, the US now spends far less on defence than during the Cold War. Military spending was 3.1% of GDP in 2025, compared with more than 13% in 1953 during the Korean War and about 9% in 1968 at the height of the Vietnam War.2
The money pays for troops, training, operations, research and weapons. In 2025, for example, the US put $9.9 billion into the Columbia-class submarine, which will carry nuclear missiles, and $13.9 billion into the Virginia-class attack submarine.1
For decades, US presidents of both parties complained that European allies spent too little. In 2014 NATO members agreed to aim for 2% of GDP; only three met that goal that year, but NATO expected all of them to reach it in 2025.6 SIPRI, which counts spending differently, found that 23 of the 32 members reached 2% in 2025.1
Together, NATO's 32 members spent $1,581 billion in 2025, 55% of all military spending on the planet; the 29 European members accounted for $559 billion of that.1 SIPRI warns that NATO's own figures are becoming harder to check, and that looser definitions invite "creative accounting": Italy reportedly tried to count the cost of a bridge to Sicily as defence-related spending.1
In June 2025 the allies went further, agreeing to spend 5% of GDP by 2035, of which at least 3.5% on core defence. European military spending jumped 14% in 2025 to $864 billion. Germany passed 2% of GDP for the first time since 1990, and Poland spent 4.5%, the most in NATO.1
US allies and partners in Asia are spending more as well. Japan raised its military budget by 9.7% in 2025 to $62.2 billion, its highest share of GDP since 1958, driven by concern about China and North Korea. Taiwan increased spending by 14% and pledged to move towards 5% of GDP by 2030.1
In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military-industrial complex.
US forces around the world
At the end of 2025 about 169,600 active-duty US troops were stationed in foreign countries, led by Japan (54,288), Germany (36,436), South Korea (23,495), Italy (12,662) and the United Kingdom (10,156). Counting reservists and Defense Department civilians, the total was 221,599.10 How many bases they use depends on what you count. The Congressional Research Service identified at least 128 overseas bases in at least 51 countries in 2024,9 while researchers using a broader definition of a "base site" counted about 750 in 80 countries and colonies in 2021.11 Explore the map to see major locations and what each one does.
Keeping the sea lanes open
Most goods traded between countries travel by ship, so a few narrow passages matter enormously. The Strait of Hormuz, between Iran and Oman, carried about 20 million barrels of oil a day in 2024, roughly a fifth of global petroleum consumption.13 Only about 7% of US crude oil imports came through it; most of that oil goes to Asia, so keeping the strait open matters even more to other economies than to America's.13
The US Navy is the largest force patrolling these routes, and by law it must keep at least 11 operational aircraft carriers.12 Through its Freedom of Navigation Program, started in 1979, the US protests maritime claims it considers excessive under international law. It does this through diplomacy and by sailing warships through the disputed waters, and it challenges claims made by allies and partners as well as by rivals.14
Guarding sea lanes can also mean fighting over them. Since early 2026 the US has been in an on-and-off war with Iran. Fighting around the Strait of Hormuz has disrupted shipping and pushed up world oil prices, and a ceasefire agreed in June broke down soon afterwards.15
The same ships and aircraft are used for disaster relief. After the March 2011 earthquake and tsunami in Japan, Operation Tomodachi involved more than 24,000 US service members, 189 aircraft and 24 ships. The Navy's 7th Fleet alone flew 160 missions and delivered 260 tons of relief supplies.20 Ships carrying helicopters can reach coastlines where ports and airports have been destroyed.
GPS: a military system everyone uses
The Global Positioning System began as a US military project to tell forces exactly where they were. It is still operated and maintained by the US Space Force. Its satellites fly about 20,200 km above Earth, and the US commits to keeping at least 24 of them working 95% of the time. In mid-2023 there were 31 operational satellites.18
The system works by timing. Each satellite carries very precise clocks and constantly broadcasts the time and its position. A receiver that picks up signals from at least four satellites can calculate where it is from the tiny differences in how long each signal took to arrive.18
For years the military deliberately blurred the civilian signal, a feature called Selective Availability. In May 2000 President Bill Clinton ordered it switched off, to make GPS more useful to civilian and commercial users worldwide. The US government says it has no intent to ever use it again.19
The result is a military system that became an everyday tool for the whole planet. GPS positioning and timing now sit inside phones, aircraft, ships, farm machinery, banks and power grids, used every day by billions of people who will never see a soldier.
