What? War is very organized dum dum.

If anything, war is a lot more organized, now.

Oh, you are serious? It has to do with communication. For a formation to be effective, it needs to be controlled centrally. Otherwise its just a mob. Back before the invention of the telegraph, the only way for a commander to keep control was to use his voice. So, the men in his command had to be within hearing distance. As communications grew more complex and miniaturized, they could be placed in smaller and smaller formations, allowing for a much more diverse strategy and looser groupings. If you look at it from a larger, more strategic altitude, the formations are just as orderly as they were back in 1776. They are just more spread out.

During the American Revolution, the commanders under Washington adapted to this new type of warfare (Washington kept the lines). This was due to size of the colonial army being so small that they couldn't afford to lose people, and the American landscape was different from Europe with it's infinite trees and hills. Early guerrilla warfare just happened.

open warfare, such as what was seen in ww2 pretty much disappeared after vietnam

Its explained in this song

Well, now we're fighting an asymmetrical war...if that's what you're asking?

With pictures.