I would keep it in 3rd person as dead people can’t write stories.  But if I wrote it in first person I would have it in the perspective of the soldier because you could see if anyone else had tried to cross the river/lake.

‘First day on the job’, I think, ‘Why am I worrying?’ I see a rifle being taken off of the gun rack. ‘Oh yeah, because my job involves shooting people.’ They hand the gun to me and a box full of ammunition. I nod to my officer and walk to my post at the bridge. Hope I don’t have to shoot anyone on my first day.