It’s my understanding that a battle is a pretty scary place to be, and if you’re not careful you might even get hurt. I know modern soldiers take this risk because of intense training, professionalism, good leadership and ideals of helping their nation or ideology. But before this period and after the fall of the Roman empire it seems like soldiers were much more mercenarily motivated and would be happy to pillage and plunder, but less willing to fight an armed and dangerous opponent. Did liquid courage play a large role in making armies actually fight each other? Is there an idea about how drunk armies would get before battles?
Alcohol has always been a major part of military history, even after rise of professional armies.
While more more infamously a naval tradition (such as the British rum ration, which lasted until the 70s), most land armies were supplied with some daily ration of alcohol, or the soldiers would find it. An infamous example which highlights this is the Battle of Carancebes (see: http://en.wikipedia.org/wiki/Battle_of_Kar%C3%A1nsebes), where a the Habsburg army started feuding over liquor and ended up charging and routing themselves. Professional armies would often use the bribe of alcohol to keep their soldiers under control, and when not in battle many would be in a state of perpetual mild drunkeness.
To answer your question more specifically, the groups that would pillage and plunder would drink, but not much before fighting, as this would massively hinder ability and reaction time. There was a military tactic known as "Chevauchee," which basically meant running around and pillaging, both to sustain your troops off the land, and to weaken your enemy. There would be significant drinking and carousing after you had managed to enter and take over a town or village, but no intelligent fighter would ever do this before battle.
While alcohol has pretty much been taken out of the modern military invasion, there is still rampant drug use; many modern soldiers (think Americans) use ephedrine-caffeine supplements; which originated for bodybuilding but have the pleasant side effect of making you hyper alert. Stimulant use started in WWII (by the Germans, mostly) and has been advanced and developed upon to this very day.