"at dawn entered Bosphorus"

The Bosphorus is a strait between European and Asian Turkey.  The capital city of Istanbul (formerly Constantinople) straddles the strait.

 

The Bosphorus (red) and the Dardanelles (yellow)
Creative Commons Attribution Share AlikeThe Bosphorus (red) and the Dardanelles (yellow) - Credit: Thomas Steiner/Wikimedia Commons