The Optimates trace their origins to around 575 AD, when Emperor Tiberius II Constantine assembled an elite regiment of foederati — foreign troops fighting under Roman contract — most likely drawn from Gothic warriors. They served as a heavy cavalry reserve, their commander holding the distinctive title of taxiarchēs. At full strength they may have numbered five thousand riders, though historian Warren Treadgold estimates the corps settled around two thousand. The chronicler Theophanes the Confessor noted that descendants of these Gothic soldiers, whom he called Gothograeci — Gothic-Greeks — were still identifiable in northern Bithynia as late as the early eighth century. Whatever their numbers, the Optimates occupied a prestigious position at the centre of the imperial military structure, a body of horsemen kept ready for the decisive moment of battle.

Power in the Byzantine Empire was always fragile, and provincial military commanders — the stratēgoi — were a perpetual source of anxiety for emperors who feared their independence. When Artabasdos, Count of the Opsician Theme, led a serious revolt against Constantine V, the emperor moved to weaken his powerful western neighbours permanently. The Optimates, who had settled in the Opsician Theme's territory, were split off into a new, separate administrative district: the thema of the Optimatoi, centred on Nicomedia (today's İzmit). Their military transformation was complete and deliberate. The new thema's four thousand men would no longer serve as fighting cavalry. Instead they formed the imperial baggage corps — the touldon — providing mule teams to haul supplies and equipment for the tagmata, the professional regiments stationed in Constantinople. Emperor Constantine VII Porphyrogennetos, writing in the tenth century, noted pointedly that the Optimatoi were not subdivided into the intermediate commands that proper military themes possessed, a structural insult that reflected their diminished status.

The territory of the Optimatoi thema was geographically intimate with the capital in a way no other province could claim. It included the peninsula directly opposite Constantinople, both shores of the Gulf of Nicomedia — the elongated inlet now called the Gulf of İzmit — and stretched eastward to the banks of the river Sangarius (today's Sakarya). Nicomedia served as its capital, a city with deep imperial roots: Diocletian had ruled the eastern empire from there, and Constantine the Great was born nearby. The region was fertile and strategically positioned, its rolling hills and forested coastlines visible on clear days from the sea walls of Constantinople. Being assigned the lowest rank in the provincial hierarchy was an administrative humiliation; living on some of the most coveted land in the empire was something else entirely.

After the Battle of Manzikert in 1071, Seljuk raiders devastated the thema's rural districts, though Nicomedia itself held out. Emperor Alexios I Komnenos, reigning from 1081 to 1118, stabilised the region with help from the armies of the First Crusade passing through. Then the Fourth Crusade shattered the empire in 1204, and Latin occupiers controlled the area for a generation. John III Vatatzes recovered it in 1240, re-establishing the thema. But by the early fourteenth century, Ottoman beylik power was pushing steadily through Bithynia. The Theme of the Optimatoi, an institution that had outlasted Gothic cavalry, imperial politics, Seljuk raids, and a Crusader occupation, finally dissolved as Ottoman rule consolidated across the region — becoming the land that would eventually anchor the eastern shore of the Gulf of İzmit.

The Optimatoi left no monuments to themselves — no towers with their name carved in stone, no inscribed dedications. What survives is the outline of their territory, which maps almost exactly onto the modern Kocaeli Province east of Istanbul. Nicomedia became İzmit, now the industrial heart of the region. The Gulf of Nicomedia became the Gulf of İzmit. The ancient road network that mule teams once trudged, hauling the emperor's campaign equipment, lies beneath highways that carry container trucks today. The province these reluctant muleteers named endured for six hundred years. That is a considerable legacy for soldiers who spent most of that time being reminded they were no longer soldiers at all.