Which military branch is considered the 'first line of defense'?

I've been doing some research on different branches of the U.S military, and it got me wondering which one is typically considered the 'first line of defense'? I know different branches have different responsibilities, but I'm curious about how they all interact in times of conflict.

The term "first line of defense" can be a little misleading, as each branch has different roles and responsibilities that can tie into this concept. However, typically the concept of a 'first line of defense' is attributed to the U.S Coast Guard. The primary reason for this designation is their roles in maritime security, drug interdiction, aids to navigation, search and rescue, living marine resources, marine safety, defense readiness, migrant interdiction, marine environmental protection, and law enforcement.

However, it's also crucial to note that the military's layered defense approach means that the different branches work cooperatively. The U.S. Space Force, for example, defends against threats in outer space. The Navy might be the first line of protection for potential maritime threats, the Air Force for air threats, and the Army for land-based threats. All these components work alongside and support one another as part of the overall defense strategy.

In the end, each branch plays a pivotal role, and the 'first line of defense' can depend on the nature of the threat at hand. It's less about a hierarchy and more about a coordinated effort among all the branches.