the men are described as "chocking with dust and red from perspiration" why were they like this?

The men have been in battle against the "blue machine" (the North). Henry talks about the dust they breathe in from the exploding ground around them. The dust is mixed with the taste of gunpowder. They also taste their perspiration streaming down their face. It is mixed with the metallic taste of blood that the perspiration runs into down their face and into their mouth.