Perhaps the most controversial discussions of the suffragettes campaigns have focussed on the so-called militant/constitutionalist debate. The WSPU took as its slogan, "Deeds not Words", and adopted the argument of the broken pane of glass as far more effective than the NUWSS's quiet lobbying. Although the suffragettes were careful to avoid loss of life, protests just prior to the First World War included pouring acid on golf courses, arson attacks and even bomb attempts. Leuchars railway station was burnt down in 1913, for example. From 1909 onwards, the Liberal government of the day made legal the force feeding of those suffragettes who went on hunger strike. In practice, of course, as historians have pointed out, the boundary between militant and constitutionalist was blurred and complex. Many women were members of both campaigning organisations and the WSPU and NUWSS supported each other's activities, at least until the escalation of the militant campaign. Moreover, what counted as militant shifted over time. In the early 1900s, for example, heckling a politician at a public meeting was regarded as militant, and women who did so would be thrown out of the meeting hall by stewards or even arrested. In a speech of 1908, Christabel Pankhurst includes among "the militant methods of women today", protesting at public meetings and marching in procession to the House of Commons. Vigorous touring and setting up suffragette stalls were the sort of militant WSPU activity that the NUWSS also adopted. Over time, and in response to the reaction of the Liberal government, militancy became more violent, mainly involving damage to property.
The usefulness of militant suffragette actions has been a contentious issue, for contemporaries and ever since. Given that by the advent of the First World War, women had still not won the vote, one might argue that both constitutional and militant methods had failed, or equally, that the Liberal goverment of the day had failed to deliver it. Moreover, it was a common pattern for suffrage bills to pass their second reading in the House of Commons, and then to be talked out by MPs with well-known anti-suffragist convictions. Therefore, to assert that it was WSPU militancy itself which hindered the cause is highly contestable.