That Fighting Knife

The US Army developed the M3 fighting knife in 1943 because they felt the troops needed a combat knife.

The M4, M5, M6 and M7 bayonets are simply M3 fighting knives with attachments for their respective guns.

The troops broke them by the case-load.

Why?

Because the troops also needed utility knives.

The USMC acknowledged this by issuing the Ka-Bar.  It's kind of a mid-point between a utility and fighting knife.

The M9 bayonet appears to have dropped all pretense of being a fighting knife in favor of utility functions, even to the point of compromising its functionality as a fighting bayonet.  It's function as a pointy thing on the end of a rifle to prod prisoners is uncompromised.

The USMC OKC-3S is more optimized for fighting.  Kind of the philosophy of the ka-bar applied to a bayonet.  Good as a fighting knife, good as a fighting bayonet, good for utility.  That it was best at some of the categories is a plus, but not part of the goals.

I don't know if the Ka-bar is still issued alongside the bayonet or not.  I know I've seen lots of pictures of Jarheads toting both.