Still, many military historians have concluded that the IRA fought a largely successful and lethal guerrilla war, which forced the British government to conclude that the IRA could not be defeated militarily. The failure of the British efforts to put down the guerrillas was illustrated by the events of "Black Whitsun" on 13–15 May 1921. A general election for the Parliament of Southern Ireland was held on 13 May. Sinn Féin won 124 of the new parliament's 128 seats unopposed, but its elected members refused to take their seats. Under the terms of the Government of Ireland Act 1920, the Parliament of Southern Ireland was therefore dissolved, and executive and legislative authority over Southern Ireland was effectively transferred to the Lord Lieutenant (assisted by Crown appointees). Over the next two days (14–15 May), the IRA killed fifteen policemen. These events marked the complete failure of the British Coalition Government's Irish policy—both the failure to enforce a settlement without negotiating with Sinn Féin and a failure to defeat the IRA.