Source: screenrant.com

Chapter Three: Broken, Fixed, Broken Again, Fixed Again

Let's talk about Secret Wars. No, not the one from 2015. No, not the one from 2004; that was Secret War.

In the original Marvel Superheroes Secret Wars, a mysterious space god called The Beyonder yanked a bunch of good guys and bad guys off to a mysterious patchwork space country called Battleworld and told them to fight each other. Toward the end, Doctor Doom absorbed The Beyonder's powers and began to destroy everything. This included punching a hole through Captain America's shield.

The heroes defeated him and gave Beyonder his powers back. As a result, the planet was irradiated with some sort of wish wave that allowed strange things to come true when people wished for them. Cap wished his shield was fixed, and it was. It shattered again after it was lost beneath the ocean for some time, but a blast from Klaw's sound cannon fused its vibranium nanostructure back together.

Thanks, Klaw!

                                Source: marvel.wikia.com

Chapter Four: John Walker

Of course, the shield wasn't always in Steve Rogers' possession.

In Reagan’s ’80s, a government board sought to make Captain America accountable to the US Government, insisting that he stop acting without their explicit approval. Steve said, "Screw that." He turned in his badge and his gun – I mean, his costume and his shield. That done, he slipped on a new, black uniform and picked up an all-silver shield courtesy of Tony Stark (Black Panther replaced that one after Tony was an a-hole to Steve during Armor Wars), and called himself The Captain.

The Captain America gear was bestowed upon John Walker, an ultra-patriotic hardline conservative who put too much faith in his government and was not a great Captain America. He eventually ceded the original Cap uniform and shield back to Steve Rogers and let him regain the identity. But John didn't leave empty handed!

He walked away with Steve's The Captain uniform, and made it his own as The USAgent.