"Base Details" launches immediately into its scathing, sarcastic criticism of the "scarlet Majors" of World War I—the military elite in charge of the British Army. The poem hinges on the opening word, "If." The speaker will be describing all the things he would do if he was a major; but, of course, he's not part of that elite group. The poem draws on Sassoon's own wartime experiences with high-ranking officials: he served as an ordinary soldier and had all too clear a sense of the difference between the lives of such soldiers and the lives of the men who commanded them.
The speaker's ironic tone is clear right from the outset in the poem's title. "Base" might be read both as a noun, as in "army base," and as an adjective, meaning lowly or depraved. And those "Details" could mean groups of soldiers (such as those under a major's command) and the details of elite military life presented by the poem. These opening puns prepare the reader for a poem full of dark, sardonic humor.
The first line is pure caricature, describing the majors as "fierce, and bald, and short of breath": short-tempered, aging, and unfit. The three caesurae in the first line give it a bumbling pace that fits with the image of these slow-moving, red-faced old men.
That image already forms a stark contrast with the image of the "glum heroes" that these majors hurry "up the line to death": that is, the young, frightened soldiers actually fighting the war on the front lines. The poem doesn't say much directly about the front-line experience, but the horrors of World War I trench combat lurk in the background, creating a juxtaposition between two very different wartime experiences.
It would be a pleasant life, thinks the speaker, to live with these "scarlet Majors" at the Base, miles away from the artillery and machine-gun fire. Here, "scarlet" has multiple possible readings. These generals live a pretty indulgent lifestyle, and it shows on their faces, which are puffy and red—unlike the gaunt, pale faces of those on the front line. But "scarlet" could also relate to the waste of young life during World War I: the spilled blood of countless soldiers and civilians. Perhaps the speaker's word choice might even evoke an unconscious blush, painting the majors with a subtle hue of shame.
Though the majors are geographically far removed from the action, their decisions have a huge impact: directing the war from afar as if they were playing a game, they "speed" all those young soldiers to their terrible fates in the trenches. They have all of the power—but suffer none of the consequences. They don't feel the weight of their decisions precisely because they live such remote, luxurious lives.
And the way the soldiers are described here makes the distance between soldiers and majors even clearer. "Glum" sounds like more than a little bit of an understatement: the soldiers aren't "glum"—which means sad or dejected—as much as terrified for their lives. And when they're described as "heroes," the word feels cheap, as though it's a word that the majors throw about insincerely to put a noble spin on the deaths of countless young men.
The end-stop at the end of line 3 creates a little silence to match with the mention of "death," and draws attention to the rhyme between that word and line 1's "breath." This supports the poem's central juxtaposition, too: while the majors keep on breathing, the "glum heroes" are sent out to die.