Slade's backstory is very similar to Captain America. He enlisted in the army at the age of 16, faking his real age just to serve his country. He served in Korea for a time, until he was relocated to Camp Washington where he met his soon to be wife, Adeline Kane. Adeline was smitten by his fighting skills and quick learning, falling in love with him and marrying him soon after. She gave him two sons.
Slade served his country well, and when an officer came to him offering a chance at a program to augment his adrenal glands in order to resist truth serums, he applied immediately.
Of course the program was a ruse and it was in truth a super soldier program. The surgery went bad and Slade fell into a coma, only to wake up with enhanced strength, speed, durability and cerebral capacity. While he reached near metahuman level, the army deemed the experiment a failure, and Slade was denied the ability to re-enlist in the army. This had Slade fall into a deep depression and accompanied with some mental drawbacks from the experiment led him to become the mercenary he is famous for being.
In the end Slade's story is the same as Captain America's, only with no happy ending.