1. breastwork (noun):

a defensive work, usually chest-high 

“Corporals and other representatives of the grimy and hot-throated men who lined the breastwork had come for each squad's portion.” (Paragraph 1)

2. thronging (present progressive verb):

crowding

3. reap (verb):

to gather or take

“He was on the verge of a great triumph in mathematics, and the corporals were thronging forward, each to reap a little square, when suddenly the lieutenant cried out and looked quickly at a man near him as if he suspected it was a case of personal assault.” (Paragraph 2)

4. compelled (past participle):

forced; driven; brought about by force

“The officer had, of course, been compelled to take his sword into his left hand.” (Paragraph 5)

5. stupefaction (noun):

overwhelming amazement

6. trident (noun):

a three-pronged weapon

“In short, this weapon had of a sudden become a strange thing to him. He looked at it in a kind of stupefaction, as if he had been endowed with a trident, a sceptre, or a spade.