There have always been tiers in military organization. In the earliest forms, that separation was based upon experience in the field and outside of it during peacetime. As time progressed, higher and higher levels of tactics and strategy developed, all of increasing complexity. Some could lead well; some could procure the items that the organization needed well; some could educate and motivate well. Some were the sons of Chieftains, and assigning them roles as officers increased their loyalty, diverted it from their own villages and fathers, and assisted the King. These original Knights might possess better equipment that protected and helped them impose their power. They also might be better educated.
In the old days, a man might easily rise into the ranks of an officer, through fortune and exploits on the battlefield. Sadly, that has gotten better and worse through time.
Some of these men were altruists, a great number actually. You might not think so, but that would be imposing your own definition of that word onto medieval men, and that will always be flawed.
Some believed in a vision. Some sought to fill their bellies. Some wanted plunder. Some sought to hasten the kingdom of heaven, though they had little concept of what that meant.
Some of them have been the greatest of my friends. There is something very noble in a willingness to sacrifice one's life for a cause. One's life is not just implying the idea of death. One's life is the totality of one's existence. For anyone who knows the military life, has family who serves, or believes in them, you know that they make sacrifices every minute at reduced wages than they could ordinarily make in any occupation. We must not ever diminish their continual sacrifice moment by moment.
The military is good at two things as Rush Limbaugh says, killing people and breaking things. Anything that detracts or diverts that focus is doomed. Rush may be right about some things. He is hopelessly wrong about others. That is the way of all people.
The idealism and patriotism of our soldiers should never be betrayed. It is so precious a gift, that it is a complete sacrilege and perversion of their trust. In a SHTF scenario, our military may be called in to reestablish order against our own citizens. This is the greatest perversion I can possibly imagine.
Moving and maintaining an army is expensive. It has bankrupted kingdoms that invested too much of their resources and people's lives into it. When the SHTF, supplies, lives, and anything ancillary to the military will be a priority. It will be a big drain on whatever available resources there remain.