League Attacks allow you and your fellow League Members to deploy powerful attacks from up to three League Members.
Only League Marshals, League Captains, Field Commanders, and Fighters are authorized to organize and launch League Attacks and manage their League Members' Units. A maximum of 3 League Members can take part in a single League Attack. League Attacks may be used to capture Settlements or attack Beacons, but can not be used to lay siege to another Castle or attack Battlegrounds.
League Attack Mechanics
Follow these steps to participate in a League Attack:
1. Appoint a League Marshal, League Captain, Field Commander, or Fighter to coordinate and launch the League Attack.
2. Have all participating members select Send Reinforcement from any interface to reinforce the attack coordinator.
3. Choose the Offensive Units required for the mission and click Send.
Once the attack coordinator has received all reinforcements needed, they may launch the attack by:
1. Choosing the Settlement or Beacon to be raided and opening the Raid window.
2. Selecting League Attack.
3. Selecting the League Members who will be participating in the League Attack (only their Offensive Reinforcements will take part in the a battle) and launching the Raid.
As with other Reinforcements, sending Offensive Reinforcements to capture other Settlements does not allow you to collect Resources from that Settlement. You can help League Members capture their Settlement, but distribution of proceeds is determined by League leadership. Only the League Attack coordinator is authorized to collect Resources from a Settlement.
Once you have sent Offensive Reinforcements to take part in a League Attack, go to the Keep and open the Tracking tab to check current information about your Units location and movement.
If you send your Units for a League Attack all losses, or Experience points will be proportionally distributed among those players who have taken part in the attack.
The maximum number of Units that can be sent determined by the Force Limit of the attack coordinator. All reinforcing Units will continue to be supported by their home Castles throughout all steps of the League Attack.
Check this Tutorial from Jesse Ray Hicks.