The Sync Attack Treaty provides protection from Alliance Sync-attacks up until you reach a specific HQ level. This means that, whenever alliance members sync an attack against your base, only 1 attacker will join the battle and the remaining marches will bounce back. This Treaty effect is only applied to Attacking Side, when the Defending player is below the specified HQ level requirement. You will still be able to join sync attacks with your allies, even if your Base is under the Sync Attack Treaty.
An automatic Protection activation will be triggered at the end of a battle, which means that bases under a Sync Attack Treaty will not suffer from multiple attacks queued up against their base.
How do I know if my base is under a Sync Attack Treaty?
While active the Sync Attack Treaty icon will be visible at the top of the screen (where your active boosters are displayed). In addition to that a Treaty icon will be displayed on your Base label next to your name, which is visible to all Commanders.
How long does the Sync Attack Treaty last?
The Sync Attack Treaty will be active until you finish the construction of HQ level 15. Whether you decide to push forward and increase your power or wait and stay under the protection for more time, its entirely up to you!