How does Longfellow's personal life factor into this poem?
Longfellow's poetry often sheds light, albeit an idealized light, on his private life. "The Day is Done" and "The Children's Hour" both throw open the doors to reveal a lovely, intimate environment in which husbands and wives and fathers and children are knit together in the closest intimacy. Though "Christmas Bells" ostensiblt concerns the most public of events—the Civil War, and the Christmas holidays—it also has an aspect of the personal to it. The loss of Fanny, Longfellow's beloved second wife, two years prior to the writing of the poem is significant, especially since his diary entries from those two Christmases express a heightened level of sorrow. Furthermore, Longfellow's son had just been wounded in war a month before he wrote the poem; this was after his son had stolen away to war without his father's permission. It makes sense that Longfellow is talking about personal grief as well as grief over the fate of the nation, and that hard as it may be, he needs to choose hope over despair.
How does Longfellow use imagery and sound to express the poem's message?
The imagery and sound of the poem helps create the contrast between the bells and the cannons of the South. The bells roll along, unbroken and in tune with the revolving of the Earth. Their tones are "wild and sweet", a "chant sublime", "loud and deep." By contrast, the cannons have gaping black maws and thunder their terrible message of hate and death. They are like an earthquake shaking and splitting the land. These images and sounds help readers feel a deeper sense of the horrors of war as well as the hope that eventually sustains.