The last image in Matthew Arnold's "Dover Beach" is a haunting image of a dark plain inhabited by clashing armies. This image is developed in the final lines of the poem:
And we are here as on a darkling plain
Swept with confused alarms of struggle and flight,
Where ignorant armies clash by night (35-37).
Essentially, in this image Arnold states that humanity inhabits a dark, vast space full of conflict and confusion. This image is the culmination of a poem that, at its heart, grapples with dwindling faith in religion and increasing uncertainty about the meaning of human existence. Arnold wrote the poem at a time when Darwin's theories of evolution were on the rise, which caused considerable doubt about religious tradition to increase. As such, the church was no longer a reliable institution of meaning for human life. In that case, it's no surprise that Arnold, lamenting the ebbing "Sea of Faith" (21), ends his poem with a pessimistic view of the confusing, dark, and precarious state of human life.