The 1916 Rising was, in military terms, a shambolic failure. Despite the fact that Britain was locked in a gruelling struggle with Germany, the Rising was still utterly crushed within a week. How then, in the aftermath of victory against Germany, did Britain fail to win the subsequent struggle with the Irish Republican Army (IRA) between 1919 and 1921? This article assesses some of the key factors that played out in the conflict, drawing particular attention to the IRA’s focus on the Royal Irish Constabulary and the consequences of this, and then later, how distorted perceptions of the proximity of success ultimately undermined British commitment. One of the most remarkable features of the conflict was the widespread belief among many on the British side (and more than a few in the Republican camp) that the IRA was on the verge of total defeat when the truce was declared in 1921. The IRA had suffered heavy casualties and were running low on weapons and ammunition. Yet, somehow the movement prevailed. This article aims to shed light on how and why that happened.