While we're all sweltering in 100-degree temperatures during this heat wave, just imagine doing the following in this heat ...
Waking up in the predawn darkness in your tent.
Having your breakfast by a fire.
Marching for the next 8 hours.
Without a break, even for lunch.
In full chain or scale armor and helmets.
Carrying your shield, spear and big sword.
All in this heat.
Then, after all that, you have to fight a battle in which you're outnumbered.
In this heat.
Now you have some idea of the plight of the poor Roman soldiers at the Battle of Adrianople in AD 378.