A worker in a nuclear plant, Leonardo discovered the unethical, illegal waste treatment and threatened the plant owners with legal action. The owners instead kidnapped Leonardo and then disposed of him elsewhere, along with the radioactive residues.
This mutated Leonardo and gave him his powers, but also disfigured him. Leonardo became an eco-terrorist, wanting to destroy the plant - which would kill many people, but also gave the world something to think. The new Iron Man (Jim Rhodes) opposed to this disguised "Atom Smasher" and, when he proved not enough, the government sent the new Firepower.
However, when Rhodes discovered Leonardo's origin and motivations, he made a deal with him: Iron Man's company Stark International, at that point chaired by Rhodes, would close down its nuclear issues and organize a media company. When Iron Man helped Atom Smasher against a too-eager, violent Firepower, at great cost to Iron Man himself, the Atom Smasher was convinced and left, giving Rhodes a chance to fulfill his promise.