The last incursion had been eight years earlier. The previous invasion had been two years in preparation. The Lamanite/Gadiantons had brought a massive army the previous time, but this time they seem to decide to erase all change of defeat. They come with “all their powers.” Since Mormon adds this phrase to his description of their virtually numberless number, he appears to have some other “power” in mind. This is perhaps a reference similar to the “exceeding great power” that we saw in the beginning of this conflict in Mormon 2:3. Perhaps the participation of Teotihuacan military forces has not been consistent, as was present in the earliest, and now in this massive (and final) incursion.
Chronological: The three hundred and seventy fifth year would be 365 A.D.