As the US aircraft carrier Dwight D Eisenhower sat moored for maintenance last week in its home port of Norfolk, Virginia, many of her crew were below decks, engaged in training for a simulated conflict in the North Atlantic.
Counterparts in the Mediterranean, Arctic and Pacific -- as well as US Navy headquarters around the world -- were fighting the same campaign, a hypothetical worldwide war that the Pentagon now believes is possible.
According to US military officials, the US Navy's "Large Scale Exercise 2023" was custom-built to push the US fleet to its limit, simulating simultaneous warfare against two major "competitors" and involving more than 25,000 personnel.
The exercise simulated everything from refuelling helicopters in the field to forcing senior commanders to decide where in the world to allocate forces while fighting two major nations at once -- and while neither "competitor" was named, the implication was Russia and China.
"Against a peer competitor that we're talking about, all the fights are going to be global," said US Navy Fleet Forces Commander Admiral Daryl Caudle, adding that this meant previous regionally focused exercises were not enough.
US Marine Corps leaders said the exercise was particularly important in forcing the Marines to work more closely with the Navy after two decades of desert wars in which the two services operated largely independently.
US officials say the Marines would likely be at the forefront of any Pacific war as they were in World War II, potentially fighting for control of strategic islands.
Such collaboration is not limited to the US Navy -- July and August also saw conferences in Europe on much more joined up thinking between the US and its Nato allies in the event of any conflict there.
The shift to this kind of thinking has been under way for at least a decade. By the end of its first term in 2012, the Obama administration had announced its "pivot" to the Pacific, while Russia's 2014 annexation of Crimea and beginning of fighting in Ukraine forced Nato to draw up its first war plans for the defence of vulnerable territory such as the Baltic states.
It has, however, been dramatically supercharged by Russia's 2022 invasion of Ukraine and growing speculation that China might make a move against neighbouring, self-ruled Taiwan in the next few years.
RELEARNING OLD LESSONS
US plans in the 2010s revolved around a war with either China or Russia. Increasingly, commanders and US strategic thinkers acknowledge they could end up fighting both at once.
In September, Russian and Chinese troops held joint exercises in Russia's Far East and the Sea of Japan involving more than 50,000 troops, 140 aircraft and 60 warships, as well as troops from several ex-Soviet nations as well as India, Laos, Mongolia, Nicaragua and Syria.
How many of those countries might genuinely side with Moscow or Beijing in any war is a very different question -- India in particular is explicitly playing a much more open game, maintaining cooperation with Russia but also a member since 2007 of the regional "Quad" alliance against China that also includes Japan, the US and Australia.
Events on the Korean peninsula might also complicate any war in Asia. North Korean leader Kim Jong-un oversaw a test of strategic cruise missiles, state news agency KCNA said on Monday, as South Korea and the United States kicked off annual military drills that Pyongyang describes as a rehearsal for war.
Tension on the peninsula has risen steadily this year, and the US-South Korean war games will be both larger than last year and include civilian civil defence drills, South Korea's first for six years.
Making tough global choices over moving military resources in a time of conflict is hardly a new skill.
Britain and France waged a global confrontation during the Napoleonic era, as did the US and Soviets during the Cold War. Few currently serving senior US military commanders have much experience of that, however -- most were only junior officers in the 1980s, and no one has experienced a shooting war on a full global scale since 1945.
The US Navy's Large Surface Exercise was designed to push US naval leaders in the Atlantic, Pacific and elsewhere through just those kinds of tough decisions, with former senior officers playing the roles of Pentagon and White House chiefs that would arbitrate and shape those choices.
PRACTISING TACTICS, FINDING FRICTIONS
At the other end of the spectrum, deploying small units to operate in locations far from their home bases was designed to detect what military planners call "frictions" -- the small inconveniences and failures that can cumulatively wreck a military operation, as Russia has sometimes found to its cost in Ukraine.
Within Europe, US and Nato air force leaders have been able to take that one stage further. Whereas once Nato's "enhanced vigilance" operations, flying US and allied combat aircraft around alliance borders, were designed primarily as a show of force, now they are also used to practise the kind of techniques flyers would use against Russian forces in any war.
At its Vilnius summit in July, Nato leaders agreed on an ambitious new set of defence plans designed to counter Russian attacks on vulnerable areas of the alliance.
Nato officials say future training exercises will be deliberately designed around those plans -- and that rather than "playing" against fictional countries, most will be designed to prepare for hostile Russian action.
Such exercises may well be useful for simulating the opening stages of any future war -- and the more realistic they are, the more useful.
Russia spent years honing its military performance in the field before the invasion of Ukraine, but analysts and observers say those drills were often too orchestrated and tightly managed to properly simulate the challenges of combat.
The larger lesson of the Ukraine war, however, may be about how long such conflicts can last, how disruptive they can be and how many resources they consume. While many Nato nations are still struggling to spend 2% of gross domestic product on defence, Ukraine is said by some estimates to have had to commit an astounding 60% of its entire economic output to fighting the war.
Even relatively localised conflict in Ukraine has yielded dramatic global shocks to food and fuel prices. Global war involving the US, Russia and China would be hugely more disruptive, even without the use of nuclear weapons.
Preparing to fight a global conflict might be vital to deterring it, but working out how many resources to commit to that -- or how realistic even the largest wargames might truly be -- is unlikely to be easy. Reuters
Peter Apps is a Reuters columnist writing on defence and security issues. He joined Reuters in 2003, reporting from southern Africa and Sri Lanka and on global defence issues. He has been a columnist since 2016. He is also the founder of a think tank, the Project for Study of the 21st Century, and, since 2016, has been a Labour Party activist and British Army reservist.