Metallurgy - which allows for alloys, so the develpment of kilns that produce and sustain enough heat for forging would have been essential. Bronze is suitable for weaponmaking, copper is too brittle in comparison. I assume, as with most inventions, experimentation would have been vital to the discovery of bronze as an alloy. (Monkeys, notably chimpansees, use sticks and stones to frighten off and/or kill enemies - and as tools. It would seem humans have learned to adapt these tools in highly innovative ways; but the discovery of making fire seems a vital development, both here and in general.)