The first sentence of "Channel Firing" spans the entire first stanza and flows into the second. It sets up the poem's main juxtaposition and its satirical tone. As warships practice firing their massive cannons out at sea, the noise travels inland to a country church. The noise is so loud that it shatters the church windows and literally wakes the dead, who sit bolt upright, thinking it is "Judgment-day"—i.e, the Apocalypse.
This dramatic irony (the dead's misunderstanding about the noise) kicks off the poem by juxtaposing the violent reality of the world with Christian expectations. That is, whereas the Christian dead think God has finally decided to intervene in worldly affairs, in reality things are the same as always. Nations are preparing for war.
The first line of the poem says a great deal about the world the dead wake up to: "That night your great guns, unawares." This "your" is striking. Because this poem is spoken by dead people, this "your" refers to the living. Specifically, it singles out the people who are responsible for testing these "great guns" out at sea: national and military leaders. More generally, however, it seems to rope in anyone who's reading the poem. That is, the "your" can be read as directly addressing the reader. This feeling that the poem may or may not be addressing the reader forces the reader to think about where they stand in relation to the issues the poem raises. Each person that reads the poem is forced to consider how they feel about war, technology, and religion.
The phrase "great guns" captures how modern countries feel about technology. Advances in weaponry is seen as positive. The guns (the giant artillery cannons on warships) are "great" not only in the sense that they are powerful, but that they are supposedly glorious. Yet the speaker immediately qualifies this phrase with the word "unawares." The adjective personifies the guns, suggesting that they—like people—can be unaware of the consequences of their own actions. The guns (and by extension those who fire them) don't realize that they've awoken the dead. So, while these guns may be "great," they're also kind of like brutes who make a lot of clumsy noise.
The first stanza establishes the form the poem will follow throughout. The first stanza is a quatrain (a four-line stanza) that follows the rhyme scheme ABAB. Metrically, every line follows iambic tetrameter, or four feet in a da-DUM rhythm. The third line exemplifies this meter well:
And broke | the chan- | cel win- | dow-squares,
Hardy often uses forms like this, which is a pretty traditional one and has often been associated with songs and folk stories. Such quatrains are good for capturing the speech and rhythms of everyday people—something the poem is clearly interested in, as all the dead seem to be regular country folk.