For the better part of two millennia, Istanbul was the most important city in the world.
The Sea of Marmara, upon which Istanbul (or Constantinople if you prefer) sits, allowed the city access to a broader region with negligible security risks. But what made Istanbul the world’s heart was trade. Land-bound West-East trade from Europe to Asia, and North-South trade from Africa and Arabia to Eurasia all transited Istanbul. So too did maritime trade from the Danube and the Black Sea to the Mediterranean. The same straits and peninsulas that provided all those trade opportunities also made Istanbul almost immune to invasion.
In time, two things gutted Istanbul’s fortunes. First, the rise of deepwater navigation c1500 allowed European-Asian trade to sail around Africa. Second, the Soviet rise after World War II turned the Danube/Black systems into internal Soviet waterways. Instead of being in the middle of everything, Istanbul found itself on the edge of nowhere.
But Istanbul can look forward to a new dawn.
The global – which is to say the global maritime – trade network is breaking down. The Russians’ hold on their near abroad also will be fracturing in the not-too-distant future. The collapse of the free trade order will push trade from the global to the regional. Russia’s fracturing will open up Turkish trade routes that have been dormant for decades. From both developments, Turkey’s standing will improve immensely.
For more information, see The Accidental Superpower, Chapters 3 and 10