Chinese Dredgers At Mischief Reef In The Disputed Spratly Islands
When is an ‘island’ not an island? When the Permanent Court of Arbitration in The Hague rules it so, and it has just ruled that many of what China calls islands in the South China Sea are in fact - rocks. 
China may well have poured concrete onto the rocks but that, in the view of the court, does not turn them into islands.
This matters because if the court had decided the other way, China would have been able to claim sovereignty and economic rights over most of the South China Sea.
Under international law a state has 200 nautical miles from its coastlines for its Exclusive Economic Zone (EEZ). Now, should Beijing seek to enforce an EEZ, other Pacific nations, including the USA, can point to the ruling to legally underpin any counter-measures they wish to take.
The case was brought by the Philippines in 2013. Manila argued that Beijing was illegally occupying seven dots of land in the Spratly Islands region of what it calls the West Philippine Sea. By so doing it argued, Beijing was in contravention of the U.N. Convention on the Law of the Sea - UNCLOS.  
The U.S Defense Department estimates that China has created at least 3,200 acres of land. It has poured sand and concrete onto the rocky outposts, levelled it out, and installed runways, military barracks, and missile launchers.  
However, the court ruled this does not make them islands and also, that because they are not capable of ‘sustaining human habitation in a natural state’, they do not qualify for the EEZ. 
China’s claims infringe upon the Philippines sovereignty and interfere with its fishing rights and potential for exploring for natural energy on the sea bed of which it is thought there may be huge reserves.
They also anger Vietnam, Brunei, Malaysia, Japan and the USA, all of which either lay claim to territorial waters, or insist on the free passage of ships through international sea lanes. 
China argues that its rights to the waters go back more than 2,000 years ago to when ships from the Han dynasty operated throughout the region. It points out that Vietnam, Malaysia and the Philippines have also created artificial islands in disputed waters. 
Beijing has what it calls a ‘nine-dash line' which, when drawn on a map, has two interpretations in Chinese government circles.
The first is that everything within the nine-dash line is Chinese. This would mean that almost the entire South China Sea belongs to China, an idea which is completely unacceptable to everyone else.
A looser interpretation is that it marks the area inside of which Chinese islands have 12 nautical miles of sovereign waters and 200 miles of EEZ. The latter interpretation holds sway for now, but this still throws up enormous difficulties for China’s neighbours, and anyone else sailing in the international sea lanes. 
This is why the Philippines brought the case and why most Pacific powers approve of the court’s decision. China immediately rejected the ruling, calling it ‘ill founded’ and says it will not be bound by it.
Beijing can now up the ante if it chooses by accelerating its island construction, declaring an ‘Air Defence Exclusion Zone’ in the skies above the disputed territory, landing fighter jets on the artificial islands it has built, and even occupying uninhabited reefs or rocks which are claimed by other countries. 
However, those would be regarded as deeply provocative moves at time when China wants stability ahead of it hosting the G20 Summit in September. 
Aggressive action by China is unlikely before then, but it is clear that China is not backing down on its claims and at some point will seek to make them an accepted reality.
That is when things could get dangerous. 
It is illegal for a foreign warship to enter the 12 miles of soverign waters from a nation's coastline without permission. If the US Navy, which guarantees that the worlds sea lanes stay open, respects the Chinese version of what is and is not sovereign territory and do not enter it, it will create a de facto acceptance.
This in turn would alarm America’s allies in the Pacific who might then bend to Beijing’s will. Conversely, if the U.S. Navy is ordered to sail right through the ‘sovereign’ territory, China then needs to make a decision about how to respond.
American warships already conduct ‘Freedom of Navigation Operations’ which skirt the boundaries of some Chinese claims. On top of this is the ever present threat of a clash at sea between fishing vessels, coastguard boats, and naval ships.
The new Philippine President Rodrigo Duterte, nicknamed ‘Duterte Harry’ is known for shooting from the hip and has threatened to ‘ride a jet ski’ into the disputed waters to make a point.
That may not come to pass, but the stage is now set for move and counter move by several different players, with the first of these coming in the autumn.
Tim Marshall is the former Foreign and Diplomatic Affairs Editor for Sky News. His most recent book Prisoners Of Geography: Ten Maps That Tell You Everything You Need To Know About Global Politics is now available in paperback.