A courageous and daring people, the Thracians were employed as mercenaries in the armies of various rulers as early as the Hellenic epoch, later in the Roman auxiliary troops, and from the second century onwards in the legions. The great slave uprising in the Roman empire (74-71 B.C.) can also be attributed to Thracian history not just because its leader and military commander Spartacus was a Thracian (it seems most likely that he came from the Medi tribe which inhabited the areas along the Strouma River) but also for the reason that most of the insurgent slaves were Thracians and Gauls. Historical chronicles on many occasions cite Thracian revolts against the Roman conquerors. The Odrysae tribe (which lived in-the Rhodope region) rebelled in the year 21 A.D., and the tribes settled south of the Balkan Range revolted in the year 26 A.D. The new ways introduced by the Romans ushered in a new stage in the development of the slave-owning society.

A great number of fortified settlements to serve as military posts for the defence of the Roman empire were constructed. Roads, bridges, public buildings, water-supply and sewage systems were constructed on a previously un-heard-of scale. What has survived of the latter, open-air theatres included, has become part of the living cities in present-day Bulgaria.

In the third century a process of decline began to take place in the life of the Roman empire. Spent in its efforts to assimilate the conquered peoples, the empire began to be influenced by the inferior cultures it had conquered. The Roman army was manned with soldiers from the rural population of the Danubean provinces. (The manning of the Roman army with Germans was to come later.) There were many Thracian cohorts in the empire. Thracian and Illyrian peasants also gained supremacy in the internecine strifes of contenders for the throne. From 236 to 238 Maximinus Thrax held the imperial throne. The Thracian armies secured the throne for Septimius Severus. The Thracian lands became the theatre of wars and conflicts.