For the life of me, I am drawing a blank on this.
What’s the joke?
What? LINK
Big strong men fleeing in terror from a spider. Ha
Maybe the spider is an invasive species doing an invasion of its kwn? Imspired by the Joro?
It’s this… the joke is this.
The first two panels make it look like they’re charging valiantly toward an enemy on the shore.
The last panel subverts this by revealing that they’re fleeing in terror from a spider on the boat.
This is the correct interpretation, imho.
Not to nitpick, but didn’t Vikings charge towards helpless victims rather than enemies? 
True - as was extensively documented by another cartoonist:
I disagree. The way I see it, the Vikings arrive with evil intent at a very small hamlet by the coast. They disembark and run towards a hamlet, attaking it. I see an attack in the second panel: look how they hold their weapons. That is not fleeing in fear. But they are defeated. Killed to the last man by the inhabitants of a defenseless, seemingly harmless hamlet by the coast.
Third panel: They will never return to the ship, and a spider makes a web on the deserted ship. The spider is a symbol of abandonement.
The joke is: Serves them right!
There is a website, appropriately named Comics I Don’t Understand (Cidu.info) that took up this particular comic a couple of weeks ago. They came up with the same few interpretations as the Dope, but it was also pointed out that this is not an unusual situation coming from Perry Bible Fellowship.
Looking like they’re attacking is the setup for the punchline.
They did both - they weren’t averse to fighting other warriors. The Great Heathen Army fought in plenty of stand-up fights.