Dover Beach
moon-blanched sand
Blanched = bleached white. Arnold's doing double work: the moon literally drains color from the sand, and 'blanched' suggests fear—the landscape itself looks drained of life.
Sophocles on the Ægean
References Antigone, where the chorus compares family curses to waves. Arnold's linking his Victorian doubt to ancient Greek tragedy—same sound, different crisis.
bright girdle furled
A girdle is a belt or sash worn around the waist. Faith used to wrap the whole earth like decorative clothing—now it's being unwrapped, leaving the world naked.
ignorant armies clash
Probably references Thucydides' account of the Battle of Epipolae (413 BC), where Athenian and Syracusan forces fought in darkness and couldn't tell friend from enemy.