The Gothic War of 535–554 was Justinian's attempt to reconquer Italy. It took nineteen years, destroyed the Ostrogothic kingdom, and ruined Italy in the process.
The Gothic War was Justinian's attempt to take Italy back from the Ostrogoths, and it is the war that broke the reconquest.
It was expected to be quick. The Vandal kingdom had fallen in nine months in 533–534, and Italy was invaded in the same confident spirit two years later.
It lasted nineteen years, consumed men and money the empire could not spare, and left Italy in a condition from which it did not recover for centuries.
Theodoric's death in 526 left the Ostrogothic kingdom with a child king and a regency, and the regent Amalasuintha was murdered by her co-ruler Theodahad in 535.
Justinian had been in correspondence with her, and her death gave him a justification. Whether he engineered the opportunity or simply seized it is not recoverable from the sources.
The African campaign had made the enterprise look cheap, and the Gothic kingdom was internally divided and led by a man his own nobles despised.
Belisarius took Sicily in 535 with almost no fighting, crossed to the mainland, and captured Naples and then Rome in 536.
Vitiges besieged him in Rome for a year and nine days and failed to take it, and the Gothic position unravelled from there.
In 540 Belisarius entered Ravenna. Offered the western empire by the Goths as a way to end the war, he appeared to accept, took the city, and handed it to Justinian. The war looked won.
It was not. Totila became king in 541 and, with a fraction of the forces the Byzantines had, took back nearly all of Italy within a decade.
Rome changed hands four times. Belisarius returned in 544 to a war that was now being fought without adequate men or money, and achieved little.
Totila repeatedly offered terms; Justinian repeatedly refused. The war ground on because neither side could finish it and one side would not stop.
In 551 Justinian finally sent an army large enough for the task, under Narses, with the money to pay it.
Narses destroyed the Gothic army at Taginae in 552 and killed Totila; he destroyed its successor and Teias at Mons Lactarius the following year.
Gothic garrisons and Frankish invasions occupied several more years, and the Pragmatic Sanction of 554 formally reorganised Italy as an imperial province.
Italy was devastated. Rome had changed hands repeatedly and was reduced to a fraction of its population; the aqueducts were cut and never fully restored; the Senate disappears from the record within a generation.
The empire had spent nineteen years and enormous resources at the same time as plague and a permanent Persian frontier, and the reconquest was never consolidated.
In 568 the Lombards invaded and took much of the peninsula against little resistance, so the Italy Justinian won was largely lost within fifteen years of winning it.