Well, since no one seems to be willing and able to help you so far, I will try. I'd like to ask a few things:

- What difficulty did you set?

To set a high difficulty is in my experience the fastest way to get war to next to all computer players. But I have to admit I did not try this out in SE V so far. It's just an assumption based on my experience with similar games.

- Have you done anything to annoy the computer player, like
- shooting its ships?
- colonizing a computer-controlled system?
- using intelligence against him?

- Does this occur any time you start a game, or just in the one you have just been playing?

In this context I have another related question to everyone who reads this:

When a computer player declares war on me, but I don't want to destroy the computer player, how can I end that war again?

I wish i could offer some assistance to both of you, however my knowledge on this is limited. I do know if you are ranked 1st by a certain amount, the AI will war declare you because your viewed as evil or something like that.

Captain Kwok will probably chime in on this when he makes his rounds, He knows the inner workings of AI.

Sorry I couldn't be more help.

Did you turn on Team Mode in the game setup? Doing so will cause all the AI players to gang up on you.

Are you significantly ahead of the other players in score?

Space Empires Depot | SE:V Balance Mod

That sounds like modding to me. Never tried that, sorry.

The difficulty does not matter; it has no effect on the game. Hostile interactions usually lead to war; but some AI's are xenophobic; and every game I play I am fighting with them. When you first meet AI's, a lot of gifts help, as does messaging them often.
To end a war, try to not enter their systems or meet their ships in combat for a while. It also helps to continue messaging them often, and sending them gifts.
Some games seem quite peaceful, while others are full of fighting. I think it depends mostly on the first few turns of contact with them and which AI you are dealing with.

My previous advice was offered thinking you were playing Balance Mod. If you are playing stock, it probably does not apply. The AI's in stock make no sense to me in what they do.