Some of mafias still blur about this. What is it exactly??? Here is Zynga definition about this new feature.
Well, some people do not like the ideas of attacking their friends. But we should remember, that is it only a game called "Mafia Wars". We will experience mafioso's life here and Declare War is one of a good example. Don't be too surprised if one of your mafia family do betrayal on you!
Apparently, you can only be in one war at once. Assisting in wars as a friend or as Top Mafia doesn't count in this.
Its better to 'Call for help' or publish that you need help with this. Else, the opponent can get more help from his friends and beat the daylights outta you. Calling for help also gives your mafia a chance to get a lil free XP.
You get XP when you participate in wars even if you lose, I think it ranges 1-6 XP. No stamina or health is lost when you attack the players who are at war and their Top Mafia. Its just a friendly feature. However, you lose stamina and health when you attack players who are not directly involved, but are just helping in wars.
The feature is still not fully functional. So, not everyone can 'declare war' successfully. It says "Thank you for trying out Declare War. Soon everyone will be able to start their own War."