1)From what i could see i would interpret this as when one nation gets involved all their allies jump in also. You could also say that this ends in a long chain with each one going after the other. ally, then opposing ally, then ally, etc.
2)Source A tells you that when someone messes with one of them everyone else comes out of the wood works to fight too.
Manifest destiny relates to wanting to expand lands. By expanding into Florida, America was gaining more land, which is the definition of manifest destiny.
