*INTRODUCTION*
According to PB: FttP, the Human Soldiers are stationed on Mars. Our knowledge of them is quite limited, so they remain mysterious. Despite being less specialized, the Human Soldiers are, in fact, more advanced than Civil Security.
Today's theory will be based on the Human Soldiers. The question we are going to be answering today is, who are the Human Soldiers? So let's jump right into the discussion, shall we?
*BACKGROUND*
The Human Soldiers are situated on a planet that is presumably Mars. They are lightly armored and don't appear to be very skilled or well-equipped, however, they seem to be quite advanced. The Human Soldiers use Correction Nine weaponry. This means that either Correction Nine sold their technologies to them after their collapse, or they already had access to Correction Nine technologies even before the faction's collapse. From a superiority point of view, determining rank within the Human Soldiers is rather difficult. But rank can be determined by the accenting of their armor. However, higher and lower ranks remain ambiguous. Human Soldiers appear to be using older Correction Nine technologies, such as electroshock and plasma cannons. These weapons are older versions of the Defibrillator and the Raygun C-01y. The Human Soldiers also seem to be well situated on Mars. Their security force is exceptional and they have several facilities. If we look at the armor of the Human Soldiers, it seems to originate from Correction Nine. Human Soldiers wear armor similar to that of the Marine in PB: FttP. The Human Soldiers wear a blue version of the Marine's signature "white" armor. Thus, both the armor and the weapons of the Human Soldiers come from Correction Nine. Accordingly, the Human Soldiers and Correction Nine have a lot in common.
Side Note: The Marine's armor in PB: FttP is basically an older variant of the Marine's armor in PB2
*THE THEORY*
Based on the information that has been provided to us and all of the points we have discussed in the previous section, we can surmise that the Human Soldiers are, in fact, part of Correction Nine, more specifically, Correction Nine guards. Correction Nine was shut down anterior to the events of PB: FttP, which means Human Soldiers were stationed on Mars before the events of PB: FttP. They most likely were there for research purposes, but what were they researching specifically? They were most likely stationed on Mars for researching Grubs. We need to dive a little deeper into the lore of Grubs. Grubs are lifeforms that are harvested and turned into battle-hardened soldiers. Though it is common for robotic parts to be implanted into Grubs' bodies, to put it in simpler terms, they are heavily modified. This explains why the Human Soldiers have multiple facilities, it is because they are conducting mass research on Grubs.
The reason why the Human Soldiers don't have newer Correction Nine equipment: Correction Nine provided the Human Soldiers with older equipment because that was the latest Correction Nine technology available at the time they were stationed on Mars. After Correction Nine was shut down, Human Soldiers weren't given the opportunities for their equipment to be updated. Therefore, Human Soldiers used the Grubs to keep them safe and expand their security force so that they don't have to wait for new and updated equipment to arrive.
The reason why the Human Soldiers attack the Marine: Human Soldiers attacked the Marine because Correction Nine was shut down before the events of PB: FttP. It is likely that the Human Soldiers were unaware of Correction Nine's plan to send the Marine and Noir Lime back in time. Correction Nine probably operated from Earth, while the Human Soldiers operate from Mars. There is obviously going to be an absence of communication between them. Additionally, it's very likely that Correction Nine kept the entire operation a secret. Even if the Human Soldiers had been aware of the operation, the Marine and Noir Lime would still have been presumed dead. As we all know, the time machine accidentally sent the Marine and Noir Lime into the Usurpation invasion, which already had been underway. As a result, communication with the duo was lost, and Correction Nine was shut down because of the absence of results. As a matter of fact, Human Soldiers aren't supposed to see Marines. The Human Soldiers attack Marines for these three reasons.
*CONCLUSION*
Based on everything that we have discussed, the Human Soldiers are Correction Nine guards whose prime duty is to safeguard the research that it's being conducted on Mars.
*DISCLAIMER*
This post is purely theoretical and speculative, not all points are going to be 100% accurate. So please, don't bust your bunkers over some of the points made in this post. Remember, "this is just a theory, a game theory".