Fleet Structs can be assigned to Defend allied Fleet or Planetary Structs. Defending serves two purposes:
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A Defender will attempt a pre-emptive Counter-Attack when the unit they are defending is targeted by an enemy Struct.

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If the pre-emptive strike fails to destroy the aggressor, a Defender will attempt to Block the incoming attack, absorbing the DMG themselves. Note: Defenders can only Block DMG for units that share the same Battleground.
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You can use the Defend ability to protect critical Structs and even stack multiple Counter-Attacks. Click on a Struct to see which unit it is defending, and which units are defending it.

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Indicates a Struct is being defended.
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Indicates a Struct is defending another unit.

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Example: Your Tank and Pursuit Fighter are defending your SAM Launcher against an attack from an enemy Stealth Bomber.

The defending Pursuit Fighter launches a pre-emptive Counter-Attack against the enemy Bomber, dealimg 1 DMG. However, your Tank does not Counter-Attack, as it cannot attack Air units like the Stealth Bomber.

The attacking Bomber survives the pre-emptive strike, and launches its attack against your SAM. However, your Tank is able to block the incoming DMG, as it shares the same Battleground as the SAM Launcher.

Finally, the SAM Launcher fires it's own Counter-Attack, dealing an additional 1 DMG to the enemy Stealth Bomber.

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