The Precursors [i]chose[/i] not to fight back against the Forerunners. They were in awe of the violence that they saw their creations were capable of, as is explained in Silentium. [quote]During our debate, the Gravemind hinted at a vast reserve of rules accumulated more than half a billion years ago, a huge library of experiences and disputes codified into the total wisdom of the Precursors. I am there. I can see it, judge it! It floods me with case history. The High Juridical was correct! Those who created us, who formulated the very concept of the Mantle, were themselves rich with distilled precedent. I can see their rules written in our genetic codes! We are creatures of Precursor law down to the very chains of molecules within us. Precursor hatred of Forerunners is central to establishing motive. They say Forerunners rose up, unprovoked, and destroyed them. The Precursors did not defend themselves. They marvelled at the power of destruction, of reorganization. Their law includes the necessity of violating the very nature of law... And so they created the Flood to allow themselves the pleasure of watching, at a later date, the progress of their most violent and aggressive creations... I detect deliberate contradiction. How can this be? Can such sublime mentality be so distorted? And yet... So rich! So infinitely deep in meaning and broad in scope, I am overwhelmed. The Gravemind studies me, loves me so intensely it will eat me, absorb me into its very centre. I twist in a spiral of laws once brilliant but now evil, cutting, carving - setting evil precedents. A shredding maze of forensic infection. No truth anywhere. All illusion! In agony. With infinite amusement, it withdraws its tendrils and my carapace is resealed. Gravemind informs me I will be delivered back to Forerunner territory, carrying a shard of itself deep in my memory. To spread fear and pain. Burn me! Extinguish my memory! I beg you! Better that Catalog never existed![/quote]

Honestly mate, rereading the Forerunner Trilogy is super rewarding. I'd recommend you reread it just cause it makes a little more sense every time. To answer your question, the Precursors BUILT. They were easily capable of destruction and fighting, but theyd been around since.. Ever really, and never had any known reasons for taking up arms. After they created the Forerunners, they decided that their Mantle of Responsibility would have to be held by another, (Humans) but in the meantime, the Forerunners shall uphold the Mantle. Forerunners were arrogant, and much to the surprise of their creators, decided to fight back the Precursors and succeeded to do so, however this changed the entire outlook of a lot of the Precursors (having their own creations turn on them) so they vowed never to build anything again but instead take revenge. So some fled to the outer reaches of the Galactic Rim, and some turned themselves into a powder with corrupted after time and became the Flood. I hope that helps (and I hope my info is correct)

i know what happens man. i read the trilogy. its just surprising to me that this super badass race got their asses handed to them by a race they made