To take part in the Clan War, tap the Clan War button in your profile to turn it green. Thus, you will make it clear to the head of the clan that you want to participate in wars. However, the chapter starting the war may choose you to participate in it, even if the Clan War button is colored red.

If you join the clan after the start of the war or if you are not chosen to participate, you can only watch it. During the war, you can even leave the clan, but your military base will be visible on the military map and may be attacked by the enemy even after that. Exiting the clan will not affect the current war, and if you return to the clan during the ongoing war, you can even carry out your attacks.

But until the current war in the previous clan ends, you will not be able to participate in another Clan War if you join another clan. By looking at your profile, other players will be able to see that you are still participating in an unfinished war. Until the end of the previous Clan War, you will not be able to take part in the new one.