Who is sent to war on the other side of the world?

Mostly men

What kind of men?

Healthy, able bodied men of ideal breeding age

Do they return as they left?

No, trauma, ptsd, miilitary mental conditioning all modify the young men. Even if they don't die, many proceed to lead chaotic and unhealthy lives. (Not all obviously)

Who stayed behind to reproduce?

The sickly, older, and the corrupt/rich enough to avoid war

Therefore more of the future generation will be born of unhealthy or sub optimal genetics.

This may not matter significantly within a single generation, but the US has been doing military campaigns on the other side of the world for almost 100 years now.

This has had a negative eugenic effect on the population. I believe this may be a large reason for the dramatic rise in genetic disorder and general troubles in the US