Alcohol and other ways of becoming intoxicated before a battle was probably very rare. Our idea of Vikings being drunk or high on mushrooms stems from later (c. 18th century) scholars. E.g. a Norwegian Priest, whose name escapes me, was the first one to suggest, that Berserks in Viking culture ate psychedelic mushrooms to reach their wild-state, and everyone simply took him on his word, even though no proof exist of this being true. This idea is thus widely discredited today. Alcohol does seem to have been very prominent in many earlier cultures - and remains as such to this day in for example Scandinavia - but in the Icelandic Sagas, which are our best sources for life in Viking Cultures, alcohol is for celebrations, feasts, post-battle, rituals etc - the same seems to be true for Celtic culture. It is not used in battle (perhaps it was a luxury and sacred product - which large and beautifully crafted vessels seem to suggest). I do not know enough about Rome to come with any real insights, but Greek soldiers were the men of the elite in the Polis, and in Classical Greece intoxication was frowned upon by polite society - not to say, that people like Diogenes did not get drunk and did whatever suited him - he did. But as a whole, there was a great deal of restraint involved in Greek manners, and their well-organized and heavily-equipped way of battling (long spears and heavy helmets and shields) would make alcohol your worst enemy on the battlefield. Overall, I think becoming intoxicated would be unhelpful in battle - especially large scale battles, but I also have this feeling, that it must have happened somewhere, otherwise, where did we get the idea from? Last: I feel I must insert a quotes from a Norse Legend about the King of Denmark forbidding beer-brewing in a time of grain-shortage - even making it a capital offence - but a man in Jutland continued to brew, saying: "Rather dead than sober"