Though an alliance had been in effect with Aragon for more than fifty years at the time of his accession, Marianus realised that the political aim of Peter IV was nothing less than the annexation of Sardinia and, following the conquest of
Alghero (1353), he parted ways with the Aragonese. He allied with the
Genoese and the Doria, then
at war with Aragon, and made himself an enemy of the Aragonese. Marianus' first directive was against Gherardo
della Gherardesca, a loyal Pisan vassal of the Aragonese. He attacked
Castel di Castro from the south but was rebuffed. He initiated a siege until his Doria allies could attack from the north. They did, and took Alghero. His armies proved successful in the field and he succeeded in expelling the Aragonese from every redoubt on the island save the stronghold of Castel di Castro. He even menaced
Sassari in 1354. Later that year, Peter IV landed on the island at
Nulauro. Marianus promptly began a guerilla war of ambushes against royal troops until, at the end of 1355, a brief peace was signed at
Sanluri by which Marianus renounced Alghero. The peace lasted two years, during which Marianus reinforced his armies and the country progressed favourably economically. In 1365, the war resumed with full force.
Pope Urban V confirmed Arborean possession of the whole of the island save Sassari, Alghero, and
Cagliari. Peter IV, however, sent a fleet commanded by
Pere de Luna to lead an Aragonese army deep into Arborea, bypassing other fortifications to assault Oristano. The Aragonese troops were trapped between the Arborean armies commanded by Marianus and his son, the future
Hugh III, and defeated. In 1368, Marianus finally occupied Sassari. He was preparing another campaign when he died in 1376. ==Legislation==