Clan Wars is a feature that was added in the Version 6.56.1 update.
Each epic two-day war is won by the clan who scores more stars! Bonus loot is your prize for winning the war. Increase the bonus by winning attacks, but you will lose most of it if your clan loses the war!
The matchmaking system in Clan Wars puts your clan against another clan of similar strength both offensively and defensively, and of the same size, rounded down to multiples of five eligible members.
Once a Co-Leader or Leader starts a Clan war search, the clan is put into the matchmaking pool, and it may take anywhere from a few minutes to a few hours for the matchmaking process to find a suitable clan in war. While the Clan War search is ongoing, Co-Leaders or Leaders can cancel the war search at any time, but once a match is found, canceling the war can no longer be done and the war enters its preparation day.
Experience level of members and number of wars won by a clan will not influence the matchmaking process whatsoever.
Strength calculations have been updated in Clan Wars matchmaking to compensate for high level walls and heroes, skeleton traps, etc.
Eligiblity of Members
There are nine different sizes of Clan Wars, starting from 10 vs 10 and increasing by increments of 5 up to 50 vs 50. The members that will go to war are determined by the Clan Leader, the leader chooses the players that will participate among all those members who opt in for Clan Wars. A larger Clan War yields more Clan XP.
If you wish to not participate in a Clan War you can Opt-Out. You can do this by visiting your Player Profile and clicking the "Clan Wars" button and it will ask you if you prefer to be in the war or not. Click "I'm Out" if you would like to be left out, click "I'm In" if you would like to be in the war. Note: Leaders can still choose players that have chosen to opt-out by default, and can always leave out players that have chosen to opt-in by default.
Members are also not eligible for a war if they have left a clan with the member currently participating in a war; this status remains until that clan finishes their war (e.g. if Player A leaves Clan A that is currently in war with 1 day left and joins Clan B, Player A will not be eligible in Clan B's wars until 1 day has passed).
Any members that join a clan that is in the midst of a war (regardless of whether the clan is in preparation day or battle day) will have to spectate regardless of trophy count.
During Preparation Day, War Bases are open for troop donation and can be redesigned using all the parts of your regular Village. These bases defend for you during war and will keep your resources and trophies perfectly safe.
Donation of Troops
During Preparation Day, anyone participating in the war can donate troops to any of the War Bases. These troops will help defend against any attacks from the enemy. Like the normal Clan Castle, participants can donate a maximum of 5-8 troops per War Base (dependent on Clan level), disregarding housing space (e.g. one could donate 1 Dragon, 2 Wizards and 2 Archers to fill up the space in a War Base with a level 5 Clan Castle), and the capacity of a player's Clan Castle in the War Base is equal to the housing capacity of the player's Clan Castle in the normal village. Any war spectators will not be able to donate troops into the War Castle.
Scouting Enemy Bases
Apart from donating troops into War Bases, the other purpose of Preparation Day is to scout enemy bases prior to the beginning of Battle Day. Both participants and spectators alike can view enemy bases, and like normal raiding, everyone in the clan cannot see the Traps, Hidden Teslas nor Clan Castle troop compositions in enemy bases. However they can view how full the enemy Clan Castles are, much like visiting a village not part of one's clan.
These 24 hours can be used to formulate strategies to 2 or 3-star enemy bases and modify army compositions for special purposes (e.g. replacing one Dragon in a mass Dragon raid for a few Hog Riders to eliminate one Air Defense to clear way for the Dragon attack).
Each participant can attack twice during Battle Day. Each participant will have a recommended base (based upon the relative strengths of your army and their defenses), but you can attack any base you like. You must attack two separate bases with your two attacks, however; you are not allowed to attack the same base twice.
Only the best attack against each enemy counts, so clan strategy is key! If two players attack the same enemy base and one person gets one star and the other gets two, the clan only receives two stars (not three). This is to prevent everyone from attacking the easiest enemy bases.
War Win Bonus
The game calculates and displays a war win bonus unique to every base. The bonus displayed is the loot you will receive if you get at least 1 star and your clan wins the war. You still get the loot even if the base was previously 3-starred. This war win bonus is 6 times the available loot from a base (e.g. if a base has 25k Gold and Elixir available in the attack itself the war win bonus for defeating that base would be 150k), and can be increased by whatever bonus your clan has as the "War bonus extra loot" perk, if your clan is level 3 or above.
You might notice that the war win bonus shown to you is not always the same as your clanmates see for the same base. Much like how raiding normally suffers a loot penalty depending on the TH levels of the attacker and defender, so do War Win bonuses. (e.g. a base 1 TH lower only offers 90% of its otherwise available War win bonus, and 3 TH lower get 25%.) The penalty is already calculated into the number you are shown, but awareness of the penalty will help you decide when to upgrade your townhall. Upgrading prematurely will make it harder to make a profit from wars.
During Battle Day, and subsequently up to five days after the war ends you can view a complete log of attacks that have happened thus far in the war. Each attack has its own separate replay, and it has its own rounded box, showing the username of both the attacker and defender, and the result (whether the defender is defeated or whether the defender successfully defends), along with the amount of time left before the war ends. Such boxes are highlighted green if the resulting attack means that your clan gains stars, red if the attack gains stars for the enemy, and grey if the attack fails to gain new stars or loses against the defense. Any attacks that involve you attacking or defending are highlighted with a yellow border.
During Battle Day and up to five days after the war ends, you are able to view the statistics of the aforementioned war. This includes the War Events (explained above), the overall War Statistics (under the tab "War Stats") and the statistics of both your clan (under the tab "My Team") and the enemy clan (under the tab "Enemy Team"). The War Stats tab show different statistics, including attacks (number won, number lost, number remaining), the breakdown of best attacks in the war, other attack stats (new stars per attack, average destruction and average attack duration) and the most heroic attack and defense for both clans.
Both the My Team and Enemy Team tabs show the attacks that each participant made, the number of defenses won against the number of defenses the participant's War Base had in total, and the number of new stars contributed to the war tally by each member.
At the end of the war, bonus loot is delivered to your Clan Castle, along with Clan XP which you can read about in Clan Perks. You can review results and battle replays after the war is over, for a time. Check your clan's War Log to review even older wars!
For a Victory you would earn the full War Win bonus. For a Draw you would earn 40% of the war win bonus, and for a Defeat you would earn only 30% of the war win bonus.
You and your clan gain all of the Clan XP that was earned throughout Battle Day, and an extra 50 Clan XP if your clan won the war. Failing to win the war does not reduce the amount of Clan XP that was earned by your clan.
Even if you have left the clan before the war ended, you can still receive the War Bonus if you had successfully attacked at least one base in the Clan War. Any spectators, however, will not receive any War Bonus whatsoever.
Every time the number of players in Clan Wars increases by 5, starting from 10 players, it would add 15 stars for 5 players to earn for each village.
As of the Halloween 2014 update, bonus loot was increased. The war win bonus used to be 5x the loot you could get from the base (e.g. you would win 250k Gold and Elixir if you beat a base containing 50k Gold and Elixir) and losing the war would award you with only 20% of the war win bonus.
The same update increased the Clan Castle's capacity to store war loot.
There is no cooldown for Clan Wars.
If you upgrade your Clan Castle and it completes during Preparation Day, then your troop capacity stays the same on the War Map, but if you scout your own war base, it will show your troop capacity increased.
Supercell created a video commercial in April 2014 that featured the Hog Rider on a piano along with his Hog, while the Wall Breaker plays the piano in the middle of a clan war. Watch the video below!