No. A support fleet participates in defense only if it is still present at the supported target when combat begins. Once it starts returning, it is no longer treated as being at the planet or moon.
How does this rule work?
This applies to both HOLD and ACS Defend. The fact that the fleet arrived earlier is not enough if its stay ended or it was recalled before the battle.
What exactly does the server validate?
Inter-War resolves participants from the current mission state at battle time. This keeps defense aligned with actual fleet presence rather than with historical arrival information.
How does this affect the mission?
When planning support, leave enough timing margin. A HOLD period that is too short or an early recall can remove the fleet from defense just before the expected attack.
When does it matter?
Defensive support is broader than offensive ACS: you may support your own target, a member of the same alliance or an accepted buddy. Permission alone is not enough—the fleet must also be on the correct object at the relevant time.
Common mistake
For defense, the state at battle start is decisive. A fleet that has been recalled or is already returning is not treated as present at the target. HOLD/ACS Defend timing therefore matters even when the mission was originally dispatched correctly.
Related FAQ: What is the difference between ACS Defend and HOLD? · Are a planet and moon the same target in ACS?