I't in the elements (second question)

Context (a film)

"The soldiers had problems with their last dog, but Rain lets them know there will be *no dissension in the ranks"*.

It is said about a dog (Rain) who participated in fighting and he is introduced to his new squad.
They had some problems with the last dog, and this one makes something clear, but what exactly?
and what does they mean in the ranks?

thank you very much!

I have one more question:
The series I'm translating is wild on set, animals participating in films.
The chapter's name is "it's in the elements".
I cannot translate it because I don't quite understand it. the elements I know refer to nature, but what does this combination: "it' in the elements" mean?

Most grateful to you