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The last stanza of Matthew Arnold's poem, "Dover Beach," is shown below. The meaning of these lines is that the poet, regardless of what he wants, has come to realize that he and his love must face the world as one: the world cannot offer anything of any more value than the exchange of love with another person. The world may seem capable of providing "joy," "love" and "light," but in truth, the world offers none of these. The truth is that this life is a place of desolation, unless two people pledge their love. For life is like battling in the dark, where there is confusion of sound ("alarms"), there is struggle and fighting an enemy one cannot see in the dark (so does one confront a friend or an enemy?). Those who attempt to do battle in the dark are ignorant.
Ah, love, let us be true
To one another! for the world, which seems
To 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 plain
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night.
The world is like a battlefield. Make note of words in the poem which support this theme: alarms, struggle, fight, armies, clash. eNotes.com's commentary on the last stanza of the poem confirms that the comparison here is of the world to a battlefield.
What the world is really like is a battlefield at night where soldiers rush about, pursuing and firing at shadows, unable to tell friend from foe; it is a dark plain 'Where ignorant armies clash by night.'
(If you break this down for a report, two lines or less should be in quotation marks, but three lines or more are indented, without quotation marks unless there is something offset in the referenced passage that already has quotation marks—such as someone speaking. In this case, quoted materials is placed within single quotation marks: 'example'.)