At the beginning of the 6th century, the troops of Justinian I invaded North Africa and Italy. Who did they fight there?

At the beginning of the 6th century, the troops of Justinian I invaded North Africa and Italy. Who did they fight there? Why did they win easy victories at first, but then get bogged down in a long unsuccessful war?

The troops of Justinian I easily defeated the barbarians (the Vandals in Africa, the Ostrogoths in Italy) until they were supported by the indigenous people. But as soon as the tax collectors came after the Byzantine soldiers, the sympathies of the locals passed to the barbarians and they began to help them in the struggle against the Byzantines.