Passage: By 1822, Burmese expansion into Manipur and Assam had created a long border between British India and the Burmese Empire. The British, based in Calcutta, supported rebels from Manipur, Assam and Arakan fleeing into British territory. Calcutta unilaterally declared Cachar and Jaintia British protectorates and sent in troops. Cross border raids into these newly acquired territories from British territories and spheres of influence vexed the Burmese. Convinced that war was inevitable, Burmese commander-in-chief, Maha Bandula, became the main proponent of offensive policy against the British. Bandula was part of the war party at Bagyidaw's court, which also included Queen Me Nu and her brother, the Lord of Salin. Bandula believed that a decisive victory could allow Ava to consolidate its gains in its new western empire in Arakan, Manipur, Assam, Cachar, and Jaintia, as well as take over eastern Bengal.In September 1823, the casus belli was Burma occupying Shalpuri Island near Chittagong, which was claimed by the East India Company.In January 1824, Burma sent one of their top generals, Thado Thiri Maha Uzana, into Cachar and Jaintia to disperse the rebels. The British sent in their own force to meet the Burmese in Cachar, resulting in the first clashes between the two. The war formally broke out on 5 March 1824, following border clashes in Arakan.

Question: What was the cause of the war?
border clashes in Arakan