The past few weeks have seen aerial duels being fought over the North Sea and a naval vessel being sunk off Scotland’s west coast. Fortunately this was not a case of hostile combat – these episodes took place during two major multinational military exercises: Exercise Atlantic Thunder and Exercise Cobra Warrior 26-2. Further north, across the Baltic Sea, High North and North Atlantic, British forces have also recently been engaged with allies in Exercise Lion Protector. Atlantic Thunder was a air and sea exercise involving US and British naval and air assets. It was designed to hone tactics, targeting and live firing. USS Klakring – a decommissioned US Navy guided-missile frigate – was sunk by missile and torpedo strikes from US vessels, while the UK’s HMS Portland and its Wildcat helicopter successfully struck the target with a Naval Strike Missile and Sea Venom missiles.US Capt. Kelechi Ndukwe, the commander of one of the US task forces engaged in the exercise – which involved aircraft, submarines and surface vessels – said it had showcased the “lethality and interoperability” of the UK and US armed forces.Cobra Warrior meanwhile, was labelled the “largest test of the UK’s homeland defence”. It involved Royal Air Force (RAF) and Nato forces from Germany, Italy, the Netherlands and the US. The RAF said that the combined forces rehearsed the “rapid integration, resilience and combat effectiveness required to deter threats, defend the homeland and, if necessary, fight as part of a Nato force”.Lion Protector was led by the UK and involved the Baltic and Nordic states, as well as the Netherlands. As part of Lion Protector, the Royal Marines rehearsed amphibious landings alongside Danish forces in the Faroe Islands. It was their first exercise in the territory since the second world war.These multinational exercises have both military and political objectives. Each are important. But the increasing frequency of multinational operations requires armed forces to exercise with partners regularly to enhance cohesion. The UK’s defence doctrine, the manual which set out the rules and principles by which the armed forces operate, recognises the need to align defence activity with allies and partners.Working togetherIn recognition of the requirement to cooperate effectively, Nato has emphasised the need for “interoperability”. The alliance defines this as “the ability of different systems, organizations, and nations to work together coherently, effectively, and efficiently”.Interoperability falls into three dimensions: human, procedural and technical. The technical element focuses on systems and equipment. The procedural aspect is centred on doctrine and tactics, techniques and procedures. The human element includes aspects such as culture, language, trust and personal relationships.While essential, interoperability has proved difficult to measure. It is often easier to identify when there are deficiencies. Nato’s strategic command has observed that it is “almost invisible” when successful. But interoperability requires continuous effort and investment to sustain and enhance.Exercises are just one mechanism used by armed forces to do this – but they are a significant one. The focus of these exercises can range from humanitarian assistance and disaster relief to high-intensity warfare.Specific exercises, including Nato’s Coalition Warrior, have been conducted to enhance technical interoperability. The exercise focuses on testing command and control technology and information technology services to ensure readiness for future operations. Issues such as data sharing and intelligence architecture are fundamental. This was demonstrated by the centrality of integrating command systems during Cobra Warrior to enable the air forces to operate cohesively.In human terms, these military exercises remain one of four key pillars of interoperability – alongside experience, education and personnel exchanges. During Cobra Warrior, Wing Commander Mathew Thornton – the station commander of RAF Cranwell where German pilots were hosted – remarked that the exercise: “provides an opportunity to strengthen our longstanding professional relationships, train alongside one another and build lasting friendships”. There were similar comments after the massive biennial Rim of the Pacific (Rimpac) exercise, involving 30 countries, in July. The vice-commander, Japan Maritime Self-Defense Force Rear Admiral Takuo Kobayashi, said that: “The daily conversations, shared experiences, and personal relationships developed throughout the exercise strengthen our ability to respond together…”Virtual cooperationThese exercises involve considerable expense at a time when the UK and many other countries are already having to dig deep to invest in equipment, personnel and technology. Britain announced late last year that it would be scaling back international training exercises. This was characterised as a re-prioritisation rather than reduction, but it shows the pressure that the UK and its allies are under. One possible way to continue to train together is with digital exercises, such as Virtual Flag. Digital exercises allow allied forces at bases around the world can participate in training simulations via the Distributed Mission Operations Center at Kirtland air force base in the US.Digital initiatives enable wider and more frequent engagement, while also enhancing interoperability in terms of ever-more sophisticated military technology in the era of artificial intelligence. These virtual exercises supplement rather than replace traditional exercises which remain invaluable. Allies that may need to fight together, will always need to train together.Steven Paget does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.