Justinian had recently recovered much of North Africa from the Vandals. That military action in 533 had been swift under his general Belisarius, and he hoped for a similar outcome in Italy. He also prepared by sending gifts of gold to the Frankish rulers, hoping they would not interfere.
Justinian sent Belisarius to Sicily. He also sent Mundus to the Gothic province of Dalmatia. Mundus was a Gothic military commander who had been allied with Theodoric the Great. After Theodoric's death he allied himself with the Byzantine Empire. Mundus was mortally wounded when a large Gothic army arrived in Dalmatia. Theodahad, the current ruler of the Ostrogoths in Italy, emboldened by this victory, imprisoned the Byzantine ambassadors. Justinian sent a new leader and army that did manage to conquer Dalmatia. The Gothic army then went from Dalmatia to Italy.
Belisarius had landed with an army and navy in Sicily in 535. The conquest of Sicily was quick and easy, and Belisarius was ready to cross to Italy. Theodohad sent an offer to Justinian: have Sicily, recognize me as the ruler in Italy, and later you can have Italy.
Belisarius made good progress, heading northward and taking Naples after only three weeks, then Rome, entering through the Porta Asinaria (see illustration). Theodahad's inability to stem this tide angered his people, who deposed him and made Vitiges king in 536. Vitiges abandoned Rome for the Gothic capital of Ravenna, married Amalasuintha's daughter Matasuntha to tie himself more closely to the lineage of Theodoric the Great, and gathered a large army to march back to Rome.
Vitiges besieged Rome and the Byzantines, who did not have enough soldiers to exit the city and drive the Goths away. The siege of Rome lasted from March 537 until March 538. How it was finally broken will be told tomorrow.