One of the biggest tactical advantages Captain America has in any given fight is his nearly unbreakable shield, which is made out of something commonly referred to as "Proto-Adamantium" (because Adamantium was later created to try to replicate Captain America's shield). It is one of the most powerful weapons in the Marvel Universe, because there are very few things in the whole UNIVERSE that can break it. However, while the shield itself is effectively unbreakable, Captain America himself is not, and in this week's debut issue of Uncanny Avengers, we see that that vulnerability belies a little-known tactical weakness that an X-Men villain brutally exploits.
Source：https://www.cbr.com/uncanny-avengers-captain-america-x-men-captain-krakoa-attack/
