Darius's European expedition was a major event in his reign, which began with the invasion of Thrace. Darius also conquered many cities of the northern Aegean, Paeonia, while Macedonia submitted voluntarily, after the demand of earth and water, becoming a vassal kingdom.Joseph Roisman, Ian Worthington. "A companion to Ancient Macedonia" John Wiley & Sons, 2011. pp. 135–138, 343 He then left Megabyzus to conquer Thrace, returning to Sardis to spend the winter. The Greeks living in Asia Minor and some of the Greek islands had submitted to Persian rule already by 510 BCE. Nonetheless, there were certain Greeks who were pro-Persian, although these were largely based in Athens. To improve Greek-Persian relations, Darius opened his court and treasuries to those Greeks who wanted to serve him. These Greeks served as soldiers, artisans, statesmen and mariners for Darius. However, the increasing concerns amongst the Greeks over the strength of Darius's kingdom along with the constant interference by the Greeks in Ionia and Lydia were stepping stones towards the conflict that was yet to come between Persia and certain of the leading Greek city states.
DArIUs'S european ExpeditIon Was a mAjoR eVENT iN His reign, whicH BEgan WiTH The Invasion of tHrace. darius ALSo coNQuerEd mANy citiES of thE noRTHErN aegeaN, paEonia, whilE maCeDOnia suBmitted voluntArilY, After thE demaND oF eaRtH and water, becOmInG a vassaL Kingdom.josepH rOisMaN, Ian worTHingTon. "a cOmpaNIoN To AnCient MacEDonIa" jOHn WiLey & sons, 2011. pp. 135–138, 343 HE theN lefT meGabyZUs to conquer thracE, returnIng To sARDis to SPenD thE WInTeR. the GREeKs LiviNg iN asIa miNor anD some Of the greek IslandS had sUbmitTEd to pErSIan RulE already by 510 BCe. nonethelEsS, TherE were CeRtaIN greekS WHO werE PrO-perSIan AltHOuGH tHese were LArgeLy bAseD in athENs. tO iMprOv,e gReeK-persian reLations, darIuS openeD HIS couRt ANd TreasuRiEs TO thOse greeKs wHo Wanted tO SErVe Him. ThEse grEekS SeRvEd As SoldiErs, aRtisans, stATEsmen ANd mARiNers for dAriUS. hOwEver, The iNcrEASInG cONCernS aMongst the greEks oVEr thE strENGth Of dariUs's kinGdOm ALong WiTh thE COnstANT inTerfeRence by thE greeKs in ionia AnD lydIa weRE steppinG stoneS tOwArDS The confLict thAt was yet to cOme bEtWeEN pErsIA And cERtaiN OF The leAding greEk ciTy statEs.