His reign, however, was almost immediately under threat, as in early 970 a large coalition of Slavic armies crossed the Balkan Mountains and invaded Byzantium; John would have to put any eastern campaign on hold. Meanwhile, in the east, the Fatimids crushed the last of the Ikhshidid resistance in Syria and consolidated their power in Cairo. John would swiftly repel the invading Slavic force at the Arcadiopolis, and, after conquering much of mainland Greece and Bulgaria in 971, the path for an eastern campaign was cleared. While John had been fighting in Bulgaria, the Fatimids managed to break into the Byzantine Empire itself and laid siege to Antioch, which had been captured from the Hamdanids six years earlier. Soon, by spring 971, an invading force of Qarmatians into Fatimid Syria forced the Egyptian army too withdraw. The failure of the Fatimids to take Antioch proved the stability of Byzantium's eastern front, and, later in 971, John planned to initiate yet another eastern campaign.