Who are the "ignorant armies"?

Arnold is probably referring to an ancient battle recounted by the Greek historian Thucydides in which the Athenians fought Syracusans at night. In the context of the poem, however, there is a much deeper meaning involved. In "Dover Beach," Arnold is lamenting the growing retreat of religious faith, especially among the educated classes, in mid-Victorian England. The poem is addressed to the speaker's wife, standing at the window while her husband ruminates on the shingle sands beneath. Their marriage provides a haven of stability amid the swirling turmoil about them. Faith, as it currently exists, can no longer provide any of that much-needed stability, so the speaker and listener must to look to themselves and their marital love to provide truth and beauty in an ever-changing world:

"Ah, love, let us be trueTo one another! for the world, which seemsTo lie before us like a land of dreams,So various, so beautiful, so new,Hath really neither joy, nor love, nor light,Nor certitude, nor peace, nor help for pain;And we are here as on a darkling plainSwept with confused alarms of struggle and flight,Where ignorant armies clash by night."

The world is dark, and the coming clash of cultures and ideas will take place in that dark. Nature can no longer provide any illumination. The lovely sea and moon that opened the poem are now also obscured by night and darkness. In the midst of all this confusion, the speaker and listener still have each other, and that's what really matters.