 Complete the following phrase:  Ever since the Macedonian king Amyntas I surrendered his country to the Persians in about 512-511 BC, Macedonians and Persians were strangers no more. Subjugation of Macedonia was part of Persian military operations initiated by Darius the Great 521486 BC. In 513 BC - after immense preparations - a huge Achaemenid army invaded the Balkans and tried to defeat the European Scythians roaming to the north of the Danube river. Darius army subjugated several Thracian peoples, and virtually all other regions that touch the European part of the Black Sea, such as parts of nowadays Bulgaria, Romania, Ukraine, and Russia, before it returned to Asia Minor. Darius left in Europe one of his commanders named Megabazus whose task was to accomplish conquests in the Balkans. The Persian troops subjugated gold-rich Thrace, the coastal Greek cities, as well as defeating and conquering the powerful Paeonians. Finally, Megabazus sent envoys to Amyntas, demanding acceptation of Persian domination, which the Macedonian accepted. Following the Ionian Revolt, the Persian hold over the Balkans loosened, but was firmly restored in 492 BC through the campaigns of Mardonius. The Balkans, including what is nowadays Bulgaria, provided many soldiers for the multi ethnic Achaemenid army. Several Thracian treasures dating from the Persian rule in Bulgaria have been found. Most of what is today Bulgaria remained firmly under the Persian sway 