Question

What was Yamamoto's objective at Pearl Harbor?

Answer 1

I would say to annihilate the US Pacific Fleet.

Explanation:

I think that Yamamoto, even if not really enthusiastic about a war against the US, knew that the major threat posed by the US was its fleet; the kind of war that he faced in the Pacific was based upon the use of warships and in particular aircraft carriers to wage war across the huge expanse of the Pacific Ocean; so by destroying the US fleet he could hope for a kind of impasse in the war allowing Japan to expand its conquests and, in the best possible outcome imaginable, to force the US to sue for a kind of settlement favorable to Japan.

Some authors relate the opinion of Yamamoto that even with the best outcome of the attack the industrial might of the US would turn, in the long run, the situation against Japan anyway.

The fact that the attack failed to destroy the most important type of warships, the aircraft carriers, of the US fleet concurred to reduce its effect in favor of Japan.

[Admiral Isoroku Yamamoto]

Another anecdote (although I do not think it is true) relates the horror of Yamamoto in realizing that the attack happened before the declaration of war thus introducing the negative moral effect that enraged the American public.