The IUA became wracked by internal disagreement during the early twentieth century, with the issue of the partition of Ireland proving to be particularly divisive. Many unionists outside Ulster became resigned to the political necessity of Home Rule, while unionists in Ulster established a separate organisation, the Ulster Unionist Party (UUP). In 1919 the IUA finally split apart with the founding of the break-away Unionist Anti-Partition League, effectively signalling the death of institutional unionism in most of Ireland. The UUP continued to operate in Northern Ireland, and would go on to dominate domestic politics there for much of the twentieth century.