War in the middle ages were brutal.
Imagine some huge army rushing towards yours as you are, too, and they have 30 pound blunt warhammers, or sharp longswords that could easily lop off your limbs. In the midst of the two armies attacking, you probably wouldn't survive the brutality, or know where you were or who was on your side. Then, some hulking, sweaty guy stabs you in the lower stomach, dragging your intestines out with his sword, but only after he has inserted the sword into the skulls of a few other people. You then lay there, dying, while getting trampled. No one can save you.
I would rather fight a war today, then live back then.