Good question.

Many people on this board think Michael "signed on" when he visited Vito in the hospital after the bodyguards were pulled. When he said, "I'm with you now, Pop," Michael meant more than just being physically present with Vito--he was through distancing himself from his family and their business. He could have envisioned, in that moment, that he would eventually be the Don. He certainly knew that, after he killed Sol and Mac, he would never be a "civilian" again. And after the war broke out, Michael might have envisioned that Sonny could be killed, and he'd have to step in. But, the reality didn't set in until Sonny actually was killed, so, Lana, you're probably right.

As we saw in a deleted scene after Connie's wedding, Vito approved of Michael returning to college, which implies that he didn't want Michael to be the eventual Don. That's reinforced near the end, when Vito says to Michael, "I never wanted this for you, Michael...Senator Corleone...Governor Corleone..." On the other hand: In the novel, Clemenza and Paulie drive Kay to her hotel after Connie's wedding. Clemenza tells her that "the old man thought that Michael was the best of his three sons, the one who would surely inherit the family business."