One year after being crowned king, he intervened in a conflict against Aonghas Óg, a noble of Clan Donald who was revolting against the rule of his father, John of Islay.
James' support for John came too late, and in 1489 Aonghas defeated his father to become the head of Clan Donald and lord of the Scottish Isles.
Before fighting could begin in earnest, Aonghas was assassinated, allowing for James and his allies to easily gain control over the Scottish Isles.
Dubh and his allies chose to launch their invasion of Scotland while camped in Ireland, where they were pledged support by the local nobility.
His force—which was still camped in Ireland, awaiting the right time to invade the Scottish Isles—was planned to be supplemented by 2,000 men provided by Henry and his Irish allies.
However, the rebel's numbers remained hypothetical; Dubh was unable to gather the full support of the Scottish Isles, and Henry's Irish allies were unprepared to raise the 2,000 men he promised.