The poem opens In media res, or mid action, which serves to confuse the reader as they initially don't know what's going on. This also mirrors the confusion of the soldier, as they are not emotionally prepared for what will come next. This may be Armitage making a societal comment, suggesting that soldiers are launched into situations they don't fully understand, and the military lack of compassion for the individuals they are risking in strategic military situations - hence, he was "possibly not" armed. By opening in media res, the poem also reflects the chaos of war and how unpredictable it is.
By mentioning that "on another occasion we get sent out", the speaker is saying this is one occasion of many, not an exceptional case but a reality the soldiers have to deal with on a day-to-day basis. Soldiers are expected to deal with repeated exposure to suffering and horror without any help.