I may have missed it, but I didn't notice anywhere a list who had declared war on you except in the first news msg you get when they do so. I assume you can attack them back without declaring, otherwise it's moot.

If your news is quite busy with market sales, defenses and spy ops it is very hard to see who has declared on you, so I was never hitting those players. Can there be a list of countries that have declard on you?

you could check your ingame news and see when and who declared on you.