"On the fire-step stand some snipers"

Sniping was very important in WWI, as opposing armies would often be at a complete standstill in their trenches; during such periods, snipers could fire at anyone who stuck his head above ground. 

At the beginning of the war, civilian hunting rifles were used as the standard military rifles were not effective at long range.  Soon, both sides developed better sniper rifles and invented periscope sights that greatly increased their ability to operate safely.  Many snipers kept tallies of the men they had killed, and they were often promoted on the basis of their scores.