According to Brueghel
when Icarus fell
it was spring
In these opening lines, the reader is given the poem's context. By making reference to Bruegel (spelled here as "Brueghel") and Icarus's fall, Williams is making the subject of the poem clear: he is writing about the Bruegel painting "Landscape with the Fall of Icarus." This might have been intuited from the title, but he makes the connection more explicit with these grounding details. At the same time, the reader can feel the speaker already tilting their focus away from Icarus. This movement is even clear from the construction of the line, as it says "when Icarus fell / it was spring" and not "Icarus fell in spring." The speaker is subtly moving into another story. While he is noting Icarus's fall, by describing the time of year and not the circumstances surrounding Icarus, he is pulling away from that image immediately.
a farmer was ploughing
his field
the whole pageantry
The second stanza begins with an entirely new image: a farmer plowing his field in the springtime. The use of the phrase "the whole pageantry" in the last line highlights the meaning inherent to this seemingly undramatic action. This idea is further unspooled in subsequent lines, but even in this introduction the reader is effectively introduced to a vivid portrait of the farmer. The "pageantry" to which the speaker refers is the loose soil being full of life in spring. Even though this "ploughing" is habitual for the farmer, it is still an event worthy (in the speaker's view) of detailed attention. Once again, Williams shifts the attention of the poem unto the farmer, and away from Icarus.
a splash quite unnoticed
this was
Icarus drowning
This final stanza drives home the poem's central point. Referring to Icarus's watery demise as "a splash quite unnoticed" reframes the commonly perceived drama in his story. As shown in previous lines, the farmer is too busy with his field to have the time or energy to even notice Icarus drowning. This moment clinches the poem's main thesis about the impact of relative perspective on a story. In the farmer's eyes, Icarus's fall is merely "a splash quite unnoticed." This final moment, along with the focus on the farmer's story, also works in tandem with the painting. The painting places the farmer in the front of the poem's perspective and drops Icarus in the corner. In the same manner, the text makes the farmer's story the primary thread and uses these final lines to show Icarus's relative unimportance in the context of this parallel moment.