Page 40 - Turkey Tour 2018 27th February (compiled)_Classical

The strategic significance of the strait was one of the factors in the decision of the Roman Emperor Constantine the Great to
found there in AD 330 his new capital, Constantinople, which came to be known as the capital of the Eastern Roman Empire.
The phrase "swim the Bosphorus" or "crossing the Bosphorus" was, and is still used to indicate religious conversion to
the Eastern Orthodox Church.
Ottoman era (1453–1922)

On 29 May 1453, the then-emergent Ottoman Empire conquered the
of ity Constantinople following a lengthy campaign wherein the
Ottomans constructed fortifications on each side of the strait,
the Anadoluhisarı(1393) and the Rumelihisarı (1451), in preparation for
not only the primary battle but to assert long-term control over the
Bosphorus and surrounding waterways. The final 53-day campaign,
which resulted in Ottoman victory, constituted an important turn in
world history. Together with Christopher Columbus's first voyage to the
Americas in 1492, the 1453 conquest of Constantinople is commonly
noted as among the events that brought an end to the Middle Ages and
marked the transition to the Renaissance and the Age of Discovery.

The event also marked the end of the Byzantines — the final remnants of the Roman Empire— and the transfer of the control
of the Bosphorus into Ottoman hands, who made Constantinople their new capital, and from where they expanded their
empire in the centuries that followed.

At its peak between the 16th and 18th centuries, the Ottoman Empire had used the strategic importance of the Bosphorus to
expand their regional ambitions and to wrest control of the entire Black Sea area, which they regarded as an "Ottoman lake",
on which Russian warships were prohibited.
[19]
Subsequently, several international treaties have governed vessels using the waters. Under the Treaty of Hünkâr İskelesi of 8
July 1833, the Bosporus and Dardanelles straits were to be closed on Russian demand to naval vessels of other powers. By
[20]
the terms of the London Straits Convention concluded on 13 July 1841, between the Great Powers of Europe — Russia,
the United Kingdom, France, Austria and Prussia — the "ancient rule" of the Ottoman Empire was re-established by closing the
Turkish Straits to any and all warships, barring those of the Sultan's allies during wartime. It thus benefited British naval power
at the expense of Russian, as the latter lacked direct access for its navy to the Mediterranean.
[21]
Sources:

https://en.wikipedia.org/wiki/Bosphorus

Compiler – Alan McGrath