I would guess that most of the cannabis that reached Europe would of arrived through the trade routes that have existed for centuries. If spices and silk reached Western Europe all the way from the far east then why not cannabis and its derivatives. I read that the Romans did smoke "hemp" for medicinal and recreational purposes, that is just the term that was used in the source I read.
The Hashasheen https://en.wikipedia.org/wiki/Order_of_Assassins
were said to have been drugged or manipulated to become Assassins in the cult of Hassan-i Sabbah from early child hood. They would be addicted and were said to consume hashish but there isn't any evidence of them consuming hashish before or during a battle/assassination. I would agree with many in here who think that the vikings didn't smoke cannabis before a battle. Maybe afterwards but I believe fighting under the influence of cannabis would be counter-intuitive. It doesn't give you courage like a few horns of mead would or perhaps not give you courage but numb you to the potentially dire consequences that are likely to befall you in the coming engagement.
In the Arabian peninsula right now many know of captagon a drug that is consumed in pill form which is a mainly amphetamine stimulant. It gives you energy for hours and keeps you awake. It will make the most stable person more prone to violent tendencies and irritability delivering them into an almost fearless state. What ever it was the Vikings were taking before battle that would morph them into their "berserker" state I would be willing to bet it 100% wasn't cannabis.
Love this thread, I am no expert in this just sharing my thoughts so as to spur the discussion.