Some of this year’s diciest diplomatic incidents have been witnessed on the high seas. In March, after the Indonesian maritime police arrested the crew of a Chinese fishing boat for illegally fishing in their waters, a Chinese coastguard ship intervened, ramming the fishing boat and allowing its still-aboard engineer to escape arrest; a diplomatic row between the countries ensued. The following month, two Russian fighter jets repeatedly swooped low, buzzing the USS Donald Cook, an Arleigh Burke-class destroyer of the US navy, as it conducted exercises in the Baltic Sea. It was an aggressive move that, unsurprisingly, was swiftly condemned by the Pentagon.

Meanwhile, in the Gulf of Guinea just off the coast of Nigeria, a spike in pirate attacks, including the kidnapping of six crew members of a Turkish cargo ship in April, has seen a mixed bag of players – including the navies of Nigeria, the US, Ghana and Togo – working together to try to improve security.

If you want an insight into what a nation’s intentions are in 2016, look to its maritime forces. While waging battles in seas and oceans may seem like an archaic pursuit from a bygone era, the world’s waters are becoming the stage on which countries are angling for supremacy in the 21st century.

“The maritime domain is becoming increasingly important,” says Peter Roberts, a senior research fellow in sea power and maritime studies at the Royal United Services Institute (Rusi) in London. “There is a massive change in the way that power is being exercised at sea, which is seeing the rise of great maritime powers in very different ways.”

The South China Sea provides a prime example: the 3.5 million sq km expanse of water is not only a vital trade passage but also thought to be chock-full of natural resources (though exactly how full is open to argument). It is also a cauldron of simmering tensions, with China claiming much of the sea as its own. As such, the country has transformed the contested reefs and rocks of the Paracel and Spratly archipelagos into bases for airstrips and all sorts of military infrastructure, much to the consternation of Vietnam, the Philippines, Taiwan, Malaysia and Brunei, who all have competing claims on said body of water. The People’s Liberation Army Navy’s (Plan) aggressive expansion has also caused unease for maritime forces further afield – particularly the US, which is concerned by what China’s maritime strength could mean for future security on the sea.

The majority of nations abide by the UN Convention on the Law of the Sea (Unclos), which is the main international treaty for keeping order. While the convention divides up maritime territory – stipulating that a nation’s territorial waters extend 12 nautical miles (22km) from its coast – it also leaves an awful lot open to interpretation. In addition, not all countries that have signed up to Unclos have actually ratified it, not least the US. These fuzzy boundaries allow countries to flex their muscles out at sea, knowingly flouting the rules without having to worry about risking a war. “Powers are signalling to each other in a very precise way pertaining to their naval positions,” says Kun-Chin Lin, a lecturer in the Department of Politics and International Studies at the University of Cambridge. “They are developing a code of conduct for encounters at sea.”

All of this has implications far beyond the South China Sea. From the Arctic to the Indian Ocean, the Baltic Sea to the Gulf of Guinea, nascent maritime flashpoints span the globe. “One finds it hard to look at a nation in the world where there isn’t a potential maritime conflict on the rise,” says Roberts. As such, many nations are earmarking funds for their naval forces. While the US is still the world’s only true maritime superpower, other states are emerging as forces to be reckoned with. China has continued to engage in aggressive expansion; money has been poured into its defence forces and by 2020 the country will be spending €229bn on its military, with a significant portion of that money believed to be going into Plan’s coffers.

Then there’s Russia, which not only has economic interests in the Arctic and a strategic presence in both the Baltic Sea and the Black Sea but also has what some experts call the second-strongest naval force in the world, with a particularly large fleet of submarines. Meanwhile, the Royal Australian Navy, Indian Navy and the Japan Maritime Self-Defense Force are all building up their own fleets and operations with an eye on nearby threats. Even countries with little interest in engaging in military activity have an incentive to improve their maritime forces because non-state actors – including pirates off the coast of west Africa and refugee smugglers in the Mediterranean – increasingly require sea policing.

Without a capable maritime force, says Rob de Wijk, the founder and non-executive director of the Hague Centre for Strategic Studies, “you leave the defence of your sea lines of communication, and consequently economic prosperity, to other countries. You become a free rider – and free riders in international politics are usually losers.”

Yet while the sea has become the setting for competing maritime powers to jockey for dominance and new powers to emerge, it has also offered opportunity for increased co-operation. Tightened alliances – through joint naval exercises and freedom-of-the-seas patrols – between countries such as the US and Vietnam, or Japan and Australia, help to not only protect common trade interests but also keep the peace.

Even China, for all its provocations in the South China Sea, has said it will join the US for naval exercises this summer. These maritime mutualities allow for a wider network of checks and balances on states that aren’t concerned about maintaining calm waters. As Roberts says, “This is what gives the world security at sea.”