The leader of the alliance decides who and how many players will play in a war. Players have to be available (in the alliance gate). The leaders chooses an even number, 10-15-20-25-30-35-40-45 or 50 players to go to war. After clicking the search button, the system will search for a matching alliance, same number of players, same range. The war is then declared... So, to answer your question, no, someone can't just declare war on you. The system matches teams who want to war....you can't choose your opponent.