Alexander had marched his Macedonian army into what is now Bulgarai towards the Danube in 335 and went on to gradually reduce any apposing Thracian forces he encountered.

One such skirmish or battle is documented on a map below, denoted by the red circle on the current Google Earth visual but first created by the Royal Geographical Society of London, Further far greater images can be seen on the image tab above.

The extract from Wiki below clearly suggests that it was prior to Alexander becoming the overall King as his father Philip was assassinated at his daughter Cleopatra's wedding in the late summer of 336, thus suggesting this was a spring early summer Balkan campaign.

However, it appears that Alexander was highly rewarded for crushing the Thracian opposition while his father was further East, as on Philips return and prior to his surmise Alexander was rewarded by having a new town in their territory named after him being that of Alexandropolis