The humans were of equal footing, they got crushed because they were fighting the flood [u]and[/u] forerunners.
English
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It was established that the forerunners were simply the superior force, the human forces never got within 15,000 ly of the Forerunner capital. They only did as well as they did because they had prepared by studying Forerunner technology as well as aligning themselves with the San Shyume. But that merely slowed their defeat.