This question is best answered using a technique called close reading. So unfortunately, if you're just looking for an answer about the poem "Naming of Parts", you're going to have to read about close reading first. But since close reading is a useful and important concept that explains so much about literature, I think you will find it worth your while!
Close reading is a way of reading a text that is completely different from how you would normally read. When you read normally, the words enter into your mind and you remember the general idea of what the words say. But when you read normally, you don't pay much attention to each individual word itself, or the order and arrangement of the words, or to the punctuation. Close reading is the opposite. When you close read, you ignore the big picture and focus on the little details. This means asking questions such as:
- What words are repeated? Why?
- What words are capitalized? Why?
- What meaning does each individual word have?
- Do any of the words have more than one meaning?
- What effect do punctuation and line breaks have on the meaning?
- What is the order of the words? What words come first? What words come last? Why?
These little details may seem insignificant. But it's the little details that contain the most information. Resist the urge to think about the big picture, at least right away. The big picture will emerge slowly from the little details.
Another difference between close reading and normal reading is speed. You may be a slow reader, you may be a fast reader, but when you read normally, your eyes move from left to right, and you don't go backwards unless you missed something. Close reading is different. Because you're focusing on all these little details, you will spend a lot of time close reading a very small passage of text. And you won't just be reading left to right: you'll have to move backwards, reread things, underline things, and pause and think about things. For this reason, close reading is typically done with a very small passage. That passage can range in size from a single sentence to a few paragraphs. But unless you have a lot of time on your hands, you aren't going to close read an entire novel.
For this reason, this answer will only focus on the fourth and fifth stanzas of "Naming of Parts." I'm focusing on these stanzas because I find them the most interesting. Here they are below:
And this you can see is the bolt. The purpose of this
Is to open the breech, as you see. We can slide it
Rapidly backwards and forwards: we call this
Easing the spring. And rapidly backwards and forwards
The early bees are assaulting and fumbling the flowers
They call it easing the Spring.
They call it easing the Spring: it is perfectly easy
If you have any strength in your thumb: like the bolt,
And the breech, and the cocking-piece, and the point of balance,
Which in our case we have not got; and the almond-blossom
Silent in all of the gardens and the bees going backwards and forwards,
For today we have naming of parts.
One last thing about close reading. Don't close read on a computer. Print out the passage you want to close read. That way, you can underline words and jot down notes. Trust me, it helps.
So now that we understand what close reading is, the next step is to actually close read the fourth and fifth stanzas of "Naming of Parts." To do this, I need to print out the passage, look at all the tiny little details, move backwards and forwards through the text, and write down my notes. I need to ask questions of the little details: what are the exact meanings of this word? Why is this word capitalized? What is the purpose of this period? I asked all of these questions, and took notes on a piece of paper. I then typed up my notes, which I reproduce below:
The word spring has several connotations: it can mean the season, it can mean to spring (i.e. to jump), or it can mean a spring (an object that if pushed or pulled, will return to its original shape).
The line "They call it easing the Spring" is emphasized by its indentation and the fact that it is at the end of the fourth stanza.
The repetition with variation of the phrase "we call this \ Easing the spring" v. "They call it easing the Spring" is interesting. The change in pronouns seems to be important.
A bolt could be interpreted as a phallic object (i.e. it looks like a penis).
A "breech" can mean a crack in something, which could be interpreted as a vulvic or yonic object (i.e. it looks like a vagina -- BTW here's a discussion of whether vagina or vulva is the more appropriate term to use in this context, and here's a Stack Exchange question about whether vulvic or yonic is the more appropriate term to use).
The phrase "rapidly backwards and forwards" is repeated twice, making it hard to miss.
The first time the phrase "rapidly backwards and forwards" is mentioned, it is in the context of instructions for how to use the "bolt". Given the symbolism of the bold and the breech, it's pretty clear that "slide it rapidly backwards and forwards" is a reference to sex.
The second time the phrase "rapidly backwards and forwards" is mentioned, it's in the context of "And rapidly backwards and forwards / The early bees are assaulting and fumbling the flowers."
Bees and flowers have a sexual relationship: bees transfer pollen from one flower to another, which allows the plants to reproduce.
Given the fact that this stanza is filled with sexual references, when I hear the words "fumbling" and "assaulting," especially after the phrase "rapidly backwards and forwards," I immediately think of how those words can describe sex.
In list item #3 I mention the change in pronouns between "we call this / Easing the spring" and. "They call it easing the Spring". The we and they creates an us and them affect in the poem: "we" is identified with the military trainees, and the pronoun "they" is used to refer to the bees ("The early bees are assaulting and fumbling the flowers / They call it easing the Spring.") and thus to nature. The effect of this is to identify the humans as separate and different from nature: nature is not considered part of the group identified with the pronoun "we".
However, the comparison is also a very strange one. Why on earth contrast human sexuality to non-human sexuality? Those are two very different things.
The poem also comments on the inadequacy of the soldiers. The poem lists "the point of balance" as an important element of the weapon (it's one of the parts that is named), but tells us that "Which in our case we have not got" the point of balance. To be balanced, one needs to be calm and controlled; not being balanced means being clumsy.
Depending on how you read the poem, it either teases or encourages the soldiers. The soldiers are told that "easing the spring... is perfectly easy / If you have any strength in your thumb." Telling someone that something is easy can encourage them (it's easy; you can do it!) or discourage them (it's easy; why can't you do it?). The poem deliberately obscures the full context of this phrase to create this ambiguity.
Nature is described as "the almond-blossom / Silent in all of the gardens." This is in direct contrast to the soldiers naming (i.e. speaking out loud) the various parts of their weapon.
The sexual references in the poem are about watching sex. With the phrase "as you see", the poem implies that someone is demonstrating how "The purpose of [the bolt] / Is to open the breech" and that we are observing. There is no indication that the narrator is doing anything, the narrator is simply watching.
The instructions (the "naming of parts") are designed for a masculine audience. The soldiers are instructed on how to use the phallic bolt ("We can slide it / Rapidly backwards and forwards") but not on the use of the yonic breech.
I want to return to the phrase "easing the Spring" which I discussed in list items #1 and #2. A spring is something that goes back and forth; given the amount of sexual references in this stanza alone, and the frequent references to "rapidly backwards and forwards (which perfectly describes a spring's motion) it seems reasonable to interpret the word "Spring" not just as a season but as another reference to sex. This interpretation is supported by the placement of the word "easing" next to the word "Spring": easing can imply slowly moving something, which is another reference to sex in the context of the phrase "We can slide it / Rapidly backwards and forwards."
I could honestly go on for at least twenty more bullet points. I'm going to stop now because this answer has to end sometime.
The close reading is finished! We have the little details. But we aren't done yet. My notes aren't coherent, and there is no overarching argument--the big picture--that can make sense of these little details. To fix this, we will use the little details as pieces of evidence to support a larger argument. What the larger argument is is up to you, but you need to support your argument with evidence from your close reading. Otherwise, it's an opinion, not an argument. The following paragraphs represent my final argument about "Naming of Parts," based on the evidence I found using close reading. (It functions as my answer to the OP's original question):
Reed's poem "Naming of Parts" contrasts nature with war as a way of critiquing the culture surrounding the military. The poem uses metaphor to make a parallel between learning to use weapons and sex. The narrator's inexperience with both subjects is evidenced by the fact that he (the soldier is only instructed in phallic weaponry) observes rather than participates. Therefore, this poem describes an initiation rite, where violence (using a weapon) is a necessary component in sexual maturity and thus adulthood. Among other things, this poem implies a commentary on the gendered violence of the military, a commentary justified by the military's sexual assault epidemic, as well as the use of systematic rape as a military tactic.
The comparison between the military and nature serves to emphasize the strangeness, alienness, and absurdity of the situation. As the military conducts its initiation rite, it is contrasted with an image of sexual relationships in nature. This image is foreign, and its foreignness is emphasized by the contrast between "we" (the soldiers) and "them" (nature); the poem supposes that nature is fundamentally separate from the soldiers. In addition, nature is "silent" and has "balance" while the soldiers are inadequate ("it is perfectly easy / If you have any strength in your thumb"). While the soldiers struggle with the initiation rite, an alternative exists. The nature portrayed in the poem doesn't need a rite: it is self-assured and doesn't need to learn because it merely exists. This contrast highlights the absurdity of the soldiers, their initiation rite, and their violence.
So that's close reading! I hope this answer was interesting, and I hope you walk away with an understanding of how close reading is a powerful tool that can answer all sorts of interesting questions about literary texts. If you have questions about any of this, please ask!
As you read my answer, you may have disagreed with some of my interpretations of the text. You may also have thought of interpretations that I completely overlooked. That's completely normal. Different people will notice different things. In my opinion, close reading is best done as a collaborative exercise: that way no stone is left unturned. If you're interested in learning more about how and why people read the same text differently, there's a great tool called "reader response criticism" that I hope to discuss on this site at a later point.
Finally, I want to emphasize that learning how to close read is like learning how to read normally. It takes time and practice. If you want to learn to close read, I encourage you to find a passage you find interesting and just try close reading. See what you can do. And keep trying. You won't get anywhere without practice.