The Ulster Defense Association (UDA) is the largest Ulster unionist militia, formed in September 1971 by Protestant Ulster Scots who sought to remain in the United Kingdom. The UDA was responsible for murdering over 400 Irish Catholics in Northern Ireland during The Troubles, with the vast majority of its victims being random people that were shot in revenge for the IRA's killings of Protestant settlers. In 1994, the UDA declared a ceasefire, and it ended its campaign in 2007; however, the UDA remained active, and it was involved in clashes with republicans after 2007. The UDA and Ulster Volunteer Force are the two largest Ulster unionist groups in Northern Ireland.