Prologue : Troy
We don't know that much about what caused the Trojan War or whether it even happened, but there's lots of evidence that historical Troy was a trading centre of huge importance to ancient Greek states.
That's because it can control a crucial ocean strait, the Dardanelles.
At its narrowest point at Canakkale the Dardanelles are just 1.5km wide, and ocean swimmers regularly hold events crossing it.
The Bosphorus in Istanbul is even narrower.
Greece's denuded soils left it dependent on grain imports from the Black Sea from quite early times, so control of these straits was an existential issue.
The origins of the (real and historical) Greco-Persian wars came with a tussle over control of the culturally Greek Ionian city-states along the Turkish coast and up to the Dardanelles.
India and East-West trade
This pattern repeats through history. India's Kerala and Tamil Nadu coasts dominated east-west trade from antiquity because they dominated another sort of strait -- the monsoon trade winds that could get sailing ships from the Red Sea to Southeast Asia.
The British Empire
The British Empire were the real experts in this business. Look at a map of the world's crucial ocean straits and it's truly astonishing how much of it was controlled by the British, from Gibraltar to the Gulf of St Lawrence, giving access to the Great Lakes and the interior of North America (note the French nipping at Britain's heels with St Pierre & Miquelon) to Central America, where despite only getting control of one big island (Jamaica) the British controlled all but a handful of the small islands from the Bahamas to Trinidad that controlled access to the Caribbean and Gulf of Mexico. The direct route on the trade winds from Africa takes you right between the British colonies of Montserrat and Dominica, where right in the middle you find Guadeloupe, controlled by -- surprise, surprise -- France.
The Straits of Hormuz
The Straits of Hormuz, through which a third of the world's seaborne oil now passes, was dominated by British-aligned Gulf emirates and the Omani sultanate
The Red Sea
And then you have the Red Sea, where the British and French contended over control of Egypt and had colonies at Aden and Djibouti on either side of the Bab el-Mandeb.
In the 19th century they had a problem in Southeast Asia, where the Dutch in Batavia (modern-day Jakarta) controlled the straits between the islands of Indonesia and charged hefty taxes on British shipping travelling between China and India.
So Stamford Raffles starts scouting out locations for a trading post and hits on an island with about 150 inhabitants at the end of the Malaysian peninsula that the Dutch and the Sultan of Johor aren't really interested in, Singapore.
The world trades by sea
It's tempting to think that in the modern world, with the rise of aviation, telecommunications, and global finance, maritime straits don't matter all that much.
I'd argue that it's close to the opposite. They matter now more than they ever did in the past.
For one thing, we're much more dependent on the constant and uninterrupted flow of goods round the world.
Two-thirds of the world's crude moves by sea. Oil demand may have peaked but right now, if you stopped that flow, economies will grind to a halt quite quickly.
The same goes for food. Only about 30 or 40 countries out of 195 in the world are self-sufficient in terms of calories. Cut off the supply of grain to the other nations and you can bring them to their knees rather quickly.
The same rule applies for other commodities, too, as well as for exports -- which most countries are going to need if they want to pay for those imported goods.
The other thing that's changed is the growth of aircraft carriers and long-range artillery. In the Age of Sail it was quite difficult to threaten an enemy ship in the ocean because it was hard to catch up with it.
The age of Gunboat Diplomacy and really being able to using warships as a maritime threat begins in the era of steamships, and accelerates with the invention of aircraft carriers that can interdict shipping within a several hundred-nautical mile radius.
A single aircraft carrier is quite sufficient to block any one of the world's major ocean straits, the widest of which are less than 100 nautical miles across.
Chinese Foreign Policy
That's where Chinese foreign policy comes in. Because of the way the Pacific and Asian tectonic plates crash into each other, there's a fence-like string of volcanic islands along the edge of east Asia.
Without touching the Asian mainland, you can make it all the way from Alaska down to Australia without crossing more than 100 nautical miles of open ocean.
It's not hard to see why China -- which depends on seaborne imports for about three-quarters of its oil and iron ore, not to mention animal feed and foreign exchange from its export trade -- is worried about that.
The Belt and Road Initiative
A U.S.-supported alliance between Japan, Taiwan, the Philippines, and Indonesia -- which isn't likely right now, but isn't something a military planner would want to rule out -- could completely cut off China from the world's oceans.
That's one of the best ways of understanding what's happening with Xi Jinping's foreign policy.
A lot of the biggest Belt and Road infrastructure projects look like an attempt to bypass the Straits of Malacca and Singapore, from an oil pipeline through Myanmar and a railway across Malaysia to the China-Pakistan economic corridor and subsidies for trans-Asian rail.
Similarly, the tensions over the South China Sea make most sense when you consider that the disputed islands and innavigable shoals like the Spratly and Paracel Islands in effect split the sea into a series of ocean straits.
And, finally, it's worth thinking about Taiwan. While the motivation for making threatening noises about invasion is clearly nationalistic, it's worth reflecting how much Taiwan controls China's gateways to the world, and even to parts of its own territory.
Taiwan has territory on both sides of the Taiwan Strait, including heavily-fortified islands just a stone's throw from the Chinese port of Xiamen.
Southeastern China is difficult, hilly country and to this day a huge amount of cargo between Shanghai and Guangdong goes by sea.
Similarly, the straits linking the Taiwanese mainland with outlying islands of Japan and the Philippines are only about 50 nautical miles wide.