That's the message you get when you have a mutual defense treaty with another nation and war is declared against them. You will get that message even if you later on go to full alliance.
The UN declares war against alot of nations, I've found. Problem is that the UN has no units or land, so they can't build any units. Not to mention that the UN goes into debt almost immediately and it only gets worse. The UN usually declares war against nations that are world pariahs and if you're playing high or very high volatility, there are a lot of world pariahs.
You do not have to go to war against whomever attacks the nation you have the mutual defense treaty with. There isn't even a diplo hit for not declaring war. However, you will get more Casus Belli against the nations that attack your mutual defense partner.
that would make sense, but lebanons current un approval is around 70-80%