I was admittedly oversimplifying things when I described Nagumo’s decision as timidity. The situation was more complicated than that. Nagumo went into the Pearl Harbor attack fully anticipating (and, to his credit, being mentally prepared for) the possibility that his task force would suffer heavy losses. When the operation ended up going fantastically well – two successful aircraft waves inflicting heavy damage to the enemy with minimal losses of their own, and with no counterattack against his ships – he found himself in the position (as one author put it) of a man who was running at a door to bash it in with his shoulder and who ended up having the door unexpectedly opened for him at the last moment. He went from being prepared lose a couple of his carriers to wanting to preserve his task force from harm…and indeed, he got all his ships back to Japan without even a scratch in their paint. Unfortunately, it was the wrong call. Nagumo had been chosen for the job because he had seniority, not because he was an aggressive commander; he dutifully did what he’d been ordered to do, but he didn’t go further.
There’s a scene in the movie Tora Tora Tora where Nagumo argues to his air commanders (who were pleading for a third strike, this one targeting Pearl Harbor’s fuel depots and shipyards) that the war is going to be long and hard and that Japan must keep its precious carriers intact for that protracted struggle. I don’t know if the scene is historically factual or not. Nagumo does have a point when he says that in the film, but he’s also missing a counterpoint: even at the risk (which we now know would have been minimal, though he had no way of knowing it) of his task force being found and attacked the the Americans, a strike against the tank farms and dockyards would still have been worth it. The combat-focused Japanese military had a surprisingly poor understanding of the importance of logistics and infrastructure…something that you can get away with in a short local war, but not in a long one (especially against the most industrialized nation on earth) across vast oceanic distances.