I was in a big war with my eastern neighbor and just had a small force on my western border.
So the western Major declared war on me. This would have been very opportunistic and smart...but:
The attack wasnt coming. I prepared for the worst and mobilized some emergency troops to hold the line until i could reinforce them.
When the time came, the anticipated attack still didnt come. So i realized he must be busy elsewhere.
I decided to go on the offensive now and barely met any resistance. Thats because he was at war with his own western Major neighbor. Which i met on the other side of the capital.
So why is a Major thats already at war (and losing it) declaring war on me? Just because im vulnerable myself? Doesnt make sense if he cannot invade me at all because his troops are busy elsewhere. So my suggestion would be looking into this

On the other hand...the other Major i met on the other side of the capital declared war on me too now. So thats a really interesting position now

And also: Is there a menu where i can see what Majors have war with each other?