Where ignorant armies clash by night.

 This line is extracted from Matthew Arnold's one of the greatest poems, "Dover Beach", published in 1867 in "New Poems".

          Here 'ignorant armies' suggest the soldiers are ignorant because they do not know the reason for their fighting. It indicates the common human beings. 'Clash' means making a loud noise or meeting in conflict and 'night' suggests darkness.

          The common people fight against each other without any reason.  They are completely ignorant or unknown about the result of this fight. They are also ignorant about the cause of their fighting.

          The poet here perhaps refers to the night battle of Epipoloe. It is an illusion to Greek historian Thucydides' account of the battle of Epipoloe (413b.c.), a walled fortress, where the Athenians fought against an army of Syracusams at night. This line