Greek and Roman writers would highlight the Thracians as some of the best mercenaries fielded in foreign armies from the mid-5th century and beyond. They would excel at hit and run tactics, harassing heavier troops and being able to defend difficult ground. This reputation would appear to be born out of their tradition of Homeric style warfare practiced in their own lands. Though we get no detailed accounts of how they fought against one another in their home territories.
Our knowledge of the Thracians in war during the 6th and early 5th centuries comes to us thanks to account revolving around the Greek and Persian wars found in the Histories written by Herodotus. This would see their history during this period told through the context of various Persian campaigns through their lands, therefore lacking the details of how they fought, though we can get an idea from their reputation as warriors in later histories.
Thracian lands would become a crossroads for Persian campaigns during the close of the 6th and opening of the 5th centuries BC. Initially Persia would attempt to expand north into Scythian lands, where a path through Thracian lands would need to be secured. Though the hold in Thrace during this period would only stay intact as long as a sizable occupying force was present. After the Scythian campaign continued attempts would be made to secure the regions close to the Persian Empires frontiers with mixed results.
The most successful period of Persian control would come during the lead up to and during the Greek and Persian Wars. A sizable Persian force would secure lands in Thrace and Macedonian paving a way forward to Greece. The Persians held enough control in Thracian coastal regions to construct their monumental engineering feats to prepare for Xerxes invasion. Though, with the ultimate Persian defeat in Greece, Thrace would once again become very inhospitable to Persians within their lands.
The Region of Thrace
Persian Campaign against Scythia
Persian Campaigns against Greece