Sentence:  In his submission to the UN, [Christof] Heyns points to the experience of drones. Unmanned aerial vehicles were intended initially only for surveillance, and their use for offensive purposes was prohibited, yet <WSD>once</WSD> strategists realised their perceived advantages as a means of carrying out targeted killings, all objections were swept out of the way. 
Meaning of the word marked with <WSD> tags:  As soon as.