The Core Worlds militaries were the greatest weapons of war constructed by our branch of humanity when The War started.  They had the largest warships, the most powerful weapons, the most advanced tanks and mechs, and the best-equipped infantry in all the worlds.  What they lacked were two things.  Combat experience and a solid officer corps to depend on.  Oh, they were well trained, but training only goes so far when the balloon goes up.  For us, it wasn’t far enough.