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The Vong can just create black holes and suck the fleet in before it reaches any planets.
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The AIs will do a lot of damage to your systems and the spartans along with the forerunners with wipe out a lot of your armys
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The Vong use organic grown technology, AI wouldn't affect it, it's essentially a brain. Mandalorians could fight Spartans, two warrior races would be pretty even. Again, Vong could just open a black hole and they're wiped out. Or Nihilus could drain the life from them all by looking in their direction.