Sgt. Al Pratt was a soldier in the World Army on the day that Earth's "wonders" were decimated by the forces of Apokolips. While that battle was waged in the U.S., Pratt led a regiment in Papua New Guinea, escorting an atomic cannon. (Earth 2 #1) His troops were attacked by Parademons and at some point, their weapon was detonated, killing everyone in the vicinity—except Pratt. Afterwards, shielded soldiers found his body lying still at the center of a giant hand print.
Five years later Pratt was a Captain in the World Army as well as their secret weapon: the Atom. The short-statured man could now grow to a height of about fifty feet. When the threat called Grundy raged in Washington D.C., the Atom was dropped from above into the war zone. His massive frame crushed Grundy, temporarily dispatching the monster. Thinking that this mission was accomplished, he moved on to a second: to recapture the World Army's AWOL agent, Hawkgirl. She protested vehemently that she would never go back with him. (#4) Before it could be resolved, Grundy returned and the villain's power brought down the Atom, who was in turn saved by Hawkgirl and the Flash. (#5)
This wasn't the end of the Army's mission, which was extended to include the apprehension of all new wonders. When the Flash returned to his home in Lansing, Michigan, he was trailed by the Army. The Sandmen attacked the Flash's mother while the Atom took on Jay himself. The Flash and his mother were saved at the last minute, by Khalid Ben-Hassin (the future Dr. Fate). (#9)
James Robinson on the Atom: "…We see part of his journey on Earth-Two is his life as a soldier. He's defending the country and the world. We see this resilient, brave character. He's short on stature but he's big of heart. He's a committed soldier that doesn't back down from any threat despite not having the degree of power that other people might have, in terms of his size." (CBR)
"You have a Golden Age Atom and then you had Atom Smasher and then you had Damage -- you had all these different versions of the same character, basically. What we're trying to do is consolidate these iconic bits of when there are multiple versions of the character into one overall character." (CBR)
Atom possesses superhuman strength, derived from atomic power. He can also grow to several times his normal height.