Fleet Structs can be assigned to Defend allied Fleet or Planetary Structs. Defending serves two purposes:
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A Defender will attempt to Block incoming DMG to the unit they are Defending, absorbing the DMG themselves. Note: Defenders can only Block DMG for allied units that share the same Battleground.
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A Defender will attempt to Counter-Attack on behalf of the unit they are Defending, if able.

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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 Tank is able to Block the incoming damage, as the Tank and SAM Launcher share the Land Battleground. However, the Tank does not Counter-Attack, as it cannot attack Air units like the Stealth Bomber.

The defending Pursuit Fighter is able to launch a Counter-Attack, dealing 1 DMG to the Stealth Bomber.

Additionally, the SAM Launcher also Counter-Attacks, dealing an additional 1 DMG to the enemy Stealth Bomber.