Book 20-21 and Patroklos' death

After reading these two books, it becomes clear just how unstoppable Achilles is on the battlefield. From saving countless warriors from death, to directly trying to hurt or scare Achilles, it's clear that many gods have a say in the fate of the battle. However, after seeing how unstoppable Achilles is, even despite the gods stepping in, I kept wondering, if Achilles is this powerful, how has the war gone on for over 9 years? After being on the battlefield for just one day, Achilles was able to kill dozens of men, and force the Trojan army back to the city. If he was able to just accomplish this all in one day, how has the battle possibly gone on for this long? The only conclusion I could come to was that the death of Patroklos completely enraged Achilles, putting him on his path of destruction. As this is the only conclusion I could come to, it became more and more clear to me just how much Patroklos meant to Achilles. If his death shook him enough to unleash his full anger, it's very clear that their relationship was more than a simple friendship. After the war was stagnant for almost a decade, Achilles was able to make more progress in a single day than all the other days, simply due to the death of Patrkolos.