text: Around 513 BC, as part of the military incursions ordered by Darius I, a huge Achaemenid army invaded the Balkans and tried to defeat the Western Scythians roaming to the north of the Danube river. Several Thracian peoples, and nearly all of the other European regions bordering the Black Sea (including parts of the modern-day Bulgaria, Romania, Ukraine, and Russia), were conquered by the Achaemenid army before it returned to Asia Minor. Darius's highly regarded commander Megabazus was responsible for fulfilling the conquest of the Balkans. The Achaemenid troops conquered Thrace, the coastal Greek cities, and the Paeonians. Eventually, in about 512–511 BC, the Macedonian king Amyntas I accepted the Achaemenid domination and surrendered his country as a vassal state to the Achaemenid Persia. The multi-ethnic Achaemenid army possessed many soldiers from the Balkans. Moreover, many of the Macedonian and Persian elite intermarried. For instance, Megabazus' own son, Bubares, married Amyntas' daughter, Gygaea; and that supposedly ensured good relations between the Macedonian and Achaemenid rulers.
  Following the Ionian Revolt, the Persian authority in the Balkans was restored by Mardonius in 492. This not only included the re-subjugation of Thrace, but also the full subordinate inclusion of Macedon into the Persian Empire. The Persian invasion led indirectly to Macedonia's rise in power and Persia had some common interests in the Balkans; with Persian aid, the Macedonians stood to gain much at the expense of some Balkan tribes such as the Paeonians and Greeks. All in all, the Macedonians were "willing and useful Persian allies." Macedonian soldiers fought against Athens and Sparta in Xerxes' army.