İzmit is a municipality and the capital district of Kocaeli Province, Turkey. Its area is 480 km2, and its population is 376,056 (2022). The capital of Kocaeli Province, it is located at the Gulf of İzmit in the Sea of Marmara, about 100 km (62 mi) east of Istanbul, on the northwestern part of Anatolia. Kocaeli Province had a population of 2,079,072 inhabitants in 2022, of whom approximately 1.2 million lived in the largely urban İzmit City metro area made up of Kartepe, Başiskele, Körfez, Gölcük, Derince and Sapanca. Unlike other provinces in Turkey, apart from Istanbul, the whole province is included within the municipality of the metropolitan center.

Diocletian, nicknamed Jovius, was Roman emperor from 284 until his abdication in 305. He was born Diocles to a family of low status in the Roman province of Dalmatia. Diocles rose through the ranks of the military early in his career, eventually becoming a cavalry commander for the army of Emperor Carus. After the deaths of Carus and his son Numerian on a campaign in Persia, Diocles was proclaimed emperor by the troops, taking the name Diocletianus. The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.