Well, shortly yes. But it's, as usual, a little more complex than that. There's one particularly famous example and that is Flora Sandes. Sandes joined a Red Cross ambulance unit that went to Serbia in 1914 and kept trying to get as close to the front as possible, despite repeated attempts to stop her. In 1915 her ambulance crew became casualties and Serbian general Miloš Vasić allowed her to be recruited into the Serbian army. She quickly rose through the ranks to corporal and then sergeant-major. She was wounded by a grenade during close quarters fighting and was awarded the Order of the Karađorđe's Star. After she was wounded she went on a fundraising tour while convalescing and then finished the war running a field hospital as she could no longer fight. She was the only officially recognised British woman to fight during the war. Have a look at Amazons and military maids : women who dressed as men in the pursuit of life, liberty and happiness by Julie Wheelwright for her.

But there were a bunch of other women close to the firing line during the war, primarily as ambulance drivers, nurses and then later as rear echelon despatch riders and telephone operators. There were some outstanding women who risked their lives and got very close to the front line, probably the most famous two are Elsie Knocker and Mairi Chisolm who worked originally with the Red Cross but later left to start their own dressing station. They raised the funds themselves and worked closely with the Belgian army, but would treat any injured soldiers.

There's quite a few of these sorts of women who had the courage to go to war, but they did tend to stick within the gender roles that were acceptable at the time.

There were other, non-British women who actively fought during the war, particularly the Russian Women's Battalions formed by the Provisional Government after the February Revolution. These were specific combat units but they had a mixed history of use and reception. Laurie Stoff has probably done the best work on this subject in They Fought for the Motherland: Russia's Women Soldiers in World War I and the Revolution