Lines 1-4, along with the title, begin to establish the poem's setting and characters. Wilfred Owen was a leading poet of World War I, and the titular "Spring Offensive" refers to a 1918 series of attacks along the Western Front of that war. Based on the approximate date given later (the month of "May"), the setting is probably northern France.
The poem describes an unnamed collective "They"—an army unit—resting up before a battle. By withholding the names and nationality of the troops, Owen makes the poem more universally relatable—and takes ordinary politics, including the cause of the conflict, out of the equation. Instead of the political disputes that sparked WWI, the poem focuses on the human drama of battle. (And, in doing so, perhaps implies that politics become irrelevant in the heat of battle.)
As the poem begins, the soldiers have "Halted" in the "shade of a last hill." They are resting up and eating on a shady hillside, which might be the "last" in terms of their mission or tour of duty (the poem takes place in the last year of WWI)—but is also the "last" hill most of them will ever see. WWI battles were exceptionally bloody, and as the poem goes on, the soldiers seem to be preparing for death. In fact, even the "shade" that falls on them might symbolize the darkness of death (or "the shadow of death," in the biblical phrase).
For now, however, the soldiers are "at ease" (a military term meaning in a resting position, with weapons lowered). In fact, most of them are "lying easy" on the slope, using each other's "chests and knees" as "comfortable" pillows. (Of course, comfort in this situation is relative!) They sleep "Carelessly," meaning, in this context, "without care or anxiety." The battle is still hours away; they are not yet tense.
These opening lines start to establish the poem's form. It's written in iambic pentameter (10-syllable lines that generally follow a da-DUM, da-DUM rhythm), which is the most common meter in English poetry. This choice lends the poem an air of traditional gravitas. However, Owen's handling of the meter is somewhat rough and unconventional, reflecting the rough conditions of war and the poetic experimentation of his "modernist" era. (More on this in the Meter section of this guide.)
The pairing of "ease"/"knees" (lines 2-3) signals that the poem rhymes, but it will not follow a consistent rhyme scheme. In that way, it's as unpredictable as the battle awaiting the soldiers.