in these lines of a poem, How did the flares announce the front lines?

Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares(2) we turned our backs
And towards our distant rest(3) began to trudge.
Men marched asleep. Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots(4)

Question

in these lines of a poem, How did the flares announce the front lines? 

Bent double, like old beggars under sacks, 
Knock-kneed, coughing like hags, we cursed through sludge, 
Till on the haunting flares(2) we turned our backs 
And towards our distant rest(3) began to trudge. 
Men marched asleep. Many had lost their boots 
But limped on, blood-shod. All went lame; all blind; 
Drunk with fatigue; deaf even to the hoots(4)