Complex sentence: The Dardanelles were fortified with German assistance, the Bosporus was secured by the presence of "Goeben" (now "Yavuz Sultan Selim") and on September 27, the Straits were officially closed to all international shipping.
Split into simpler sentences: The Dardanelles were fortified with German assistance and the Bosporus was secured by the presence of "Goeben" (now "Yavuz Sultan Selim"). On September 27th, the Straits were officially closed to all international shipping.