In the context of the poem, the noble foe represents a worthy adversary who fought bravely and with honor. The protagonist acknowledges the strength and courage of her husband's killer and expresses admiration for their qualities, even though they are ultimately enemies on opposite sides of a conflict.
By portraying the enemy soldier in this way, Tennyson creates a sense of respect and empathy between the two sides of the conflict. It suggests that even in the midst of war, there can be recognition of the humanity and valor of one's opponents.
The use of the phrase "noble foe" also adds a layer of complexity to the poem's exploration of themes such as grief, loss, and the futility of war. It highlights the tragic nature of conflicts where individuals with admirable qualities can still become adversaries due to external circumstances.