The Scythian Expedition
Darius embarked on what is commonly called the 'Scythian expedition' or the first historic attack of Asia upon Europe which he lead in person. We have to rely almost solely on Herodotus for our knowledge of events of this campaign.
Darius' objectives seem to have been to:
-Greater access to the surrounding cities around the Black Sea
-Subdue the Scythian tribes which had made continual raids into his territory.
-Establish a foothold for further expansion into Europe.
-Access to the minterals, metals and forest deposits.
The expedition was carefully prepared, utilising both land and sea forces drawn from all parts of the empire. According to (Herodotus 4.88), Darius assembled a fleet of 600 ships and an army of 700,000. Since Darius took personal command of the expedition the forces would have been considerable, but Herodutus' numbers are considered an exaggeration. As the fleet was drawn solely from the Ionian Greeks, the actual numbers are thought to be 200 - 300 ships and probably closer to 1/10th the number of troops or 70,000 men under arms.
Darius marched with the army from Sardis to Chalcedon on the Bosphorus. It is here he met up with the fleet and crossed over the bridge of boats that had been constructed in advance. The bridge had been designed by Mandrocles of Samos and built by the Greeks from Ionia, Aeolia and the Hellespont. The fleet was then ordered to sail into the Black Sea and then to construct a bridge over the Danube river and await Darius' arrival. It could be assumed that Darius had kept the goal of his expedition a secret. The fleet reached the Danube and sailed up the river for two days until where the main stream divided and there built a bridge.
Darius marched with the army through Thrace, subduing the Gatae, a Thracian tribe after fierce fighting and placing them in slavery. Other Thracians including the Salmydessus, Scyrmiadae and Nipsaeans surrendered without fighting. Darius forced some or possibly all of the remaining Gatae to accompany him the army as he continued the march to the Danube.
It appears that Darius' intention after crossing the bridge that all the troops from the fleet join his march into Scythia. After receiving advice from a Lesbian named Coes. Darius left the bridge intact and with a large force to guard it. It would seem that it was not his original intention to return by the same route. He left orders that the Greeks were to guard the bridge for sixty days after which they were free to sail home.
The Greeks guarding the bridge and Darius' lifeline, were incited both by Miltiades, the tyrant of the Thracian Chersonese, and also the Sycthians, to unmoor their ships and sail home. The reasons cited that they chose to remain was that the destruction of the Darius' army and the subsequent revolts of the Ionian cities would not have guaranteed that the ruling tyrants would maintain their positions. It is not mentioned, but I do not doubt that Darius did not take his own precautions to preserve his line of retreat. If Darius did not actually hold any hostages, the lives of the fellow Greeks in the Persian army would have been in danger if the bridge was destroyed.
The Persians marched through the steppes but encountered little. Herodutus, says the Scythians retreated before the army, driving off their herds and destroying crops, blocking up wells and springs and burning anything of use.
The Scythian nomads were not unable to defeat the Persian army in a straight fight, although the cavalry were equal or better than the Persian cavalry, they had no answer to the Persian infantry and refused to stand and fight.
Darius was not unable to force a decisive battle with the Scythians. Lacking food and continually harassed, he was forced to retreat back to the bridge of boats which the Ionian allies were guarding. Darius is said to have left behind the sick and wounded in his panic, but considering that Megabazus went on to successfully campaign in Thrace with the pick of the troops, Herodotus may have also exaggerated Darius retreat as a panic.
Darius also was further embarrassed by the destruction of the Bosphorous bridge and was forced to find an alternate location to cross back into Asia.
Darius retreated to Sardis and never again campaigned in person, but the expedition was not a total failure. The Scythians now stopped their constant raids into Media. The Persian army now had another source of mercenaries and the general Megabazus proceeded to conquer most of Thrace and formed an allegiance with Alexander, king of Macedonia.