In order you could participate in a cartel war, the following requirements should be met:
1. Go to the cartel Options menu and check if your participation status is ON
2. Check your rating: if in your opponent cartel there are fewer members participating in war than in your cartel, those with the least rating will be excluded from the war
Attention! If you change your participation status during a war or a war search, this change will be applied only for the next war, not the current one!