The speaker in "Winter Song" uses alliteration to strengthen their promise of love. Shared sounds intensify the poem's language, making the poem—and the speaker's declarations—more musical and memorable.
In line 1, for example, repeated /m/ sounds make the speaker's command seem all the more insistent and purposeful:
Ask me no more, my truth to prove,
After that, the poem enters a hypothetical situation in which the speaker and their lover are condemned to "exile" in a harsh winter world. Alliteration helps create a vivid picture of this inhospitable environment. Take line 8, where the speaker presents the landscape as untameable and unforgiving:
Where all is wild and all is waste.
Those whooshing /w/ sounds evoke the rush of a "wild" wind whipping through a wasteland.
Later, the speaker outlines multiple threats the couple might meet in this world (and how they would rise to the challenge). The fricative /f/ sounds in "fight a fiercer race" come across as aggressive and sudden, while the /h/ sounds of "hand the hunter's spear" seem almost breathless.
Note, too, the crisp, plosive /p/ sounds of "provide," "palaces," and "pride" in lines 19-20. These sharp, popping sounds add a haughty dismissiveness to the speaker's tone, reflecting how little the speaker cares for fancy mansions when they've got a humble home with their beloved right there.