Oh, ok. This detail is not really obvious.
It initially thought it means “skip quest” and that you maybe even go a different “branch” in the quest list, or that the next quest will be different or something like that. Turns out it was much simpler. Well, I'm okay with that. But it seemed like a bug to me.
Maybe you could show a pop-up window after clicking that explaining that the player is now in “just for fun” mode, and victory will not advance you to the next quest. Some alternate storytelling for this decision (including victory) would be nice to have, so it doesn't seem too random.
Relatated minor issue: After declaring war, mission objective is not shown (unlike in all other quests). Luckily you can look it up in the pause menu if you forgot (i.e. after loading).
EDIT: I just remembered that when declaring war, the other team becomes your ally! That's quite interesting since in the other (the “normal” story path) they are your enemies because they want to see cash. I am sure a bit of storytelling could explain why.