The first four lines of “The Charge of the Light Brigade” establish the poem’s form and hint at its broader themes. The poem begins by describing a cavalry charge—six hundred British cavalrymen, the members of the "Light Brigade," ride “half a league” (about a mile and a half) into the “valley of death.” This is an allusion to a real cavalry charge that happened during the Crimean War (1853-1856). During the Battle of Balaclava (1854), a British cavalry regiment charged against a well-fortified Russian artillery position; most of the cavalrymen were killed in the assault. The attack caused an outcry in England—especially because it seemed like the order to charge was given in error, a result of miscommunication between British leaders.
As the cavalry ride "half a league, half a league," they thus ride toward their deaths. The speaker hints at this by alluding to a passage from the Bible's Psalm 23: “though I walk through the valley of the shadow of death, I will fear no evil.” The “valley of Death” that the speaker mentions in line 3—and which becomes one of the poem’s refrains—is thus a place of fear and temptation. In the psalm, the speaker’s faith in God protects him or her from the terrors of the “valley of the shadow of death.” The cavalry faces a similarly stark and dire terror as they charge against the enemy artillery. And they withstand this terror, the speaker will eventually argue, through their unusual bravery.
“The Charge of the Light Brigade” gives its reader an immediate, intimate sense of what this cavalry charge felt like, sounded like. For instance, its meter—dactylic dimeter—sounds like the clip-clop of galloping horses (recall that a dactyl consists of an accented syllable followed by two unaccented syllables; "All in the | valley of ..."). However, the speaker uses a lot of metrical substitutions to convey the chaos of the charge. Indeed, none of the opening four lines of the poem are metrically regular. And though the poem does occasionally use rhyme, it is similarly unpredictable, appearing at key moments in the poem and then disappearing again: none of these opening lines rhyme (though “onward” and “hundred” might be described as a slant rhyme).
The poem further captures the sound of the galloping horses with its use of epizeuxis, repeating the phrase “half a league” three times in the first line and a half of the poem. Working with the uneven meter, this repetition captures the rhythmic pounding of the light brigade’s horses as they charge the enemy lines.