Slitherine

I have both victory objectives and bonus objectives (dotted line hex), but I don't see any bonus/difference when I get to Kharkov...

The bonus is required to take the ahistoric path after Stalingrad.
To invade the USA you need the bonus from Moscow, the bonus from Stalingrad, and to spend prestige to take the 'Winning Path'.

Thanks! Is there a campaign tree graphic somewhere?

Google is your friend: https://static.wikia.nocookie.net/panze ... 0326170036 Note that blue scenarios indicate a branching choice. In the historical branch, you can, oddly enough, pick your flavour of final defeat at each step of the way. I say this is odd because if your nation is losing the war, you should technically have less about where you and your battle group are deployed...

One other caveat--to take the ahistorical branch (post Stalingrad and/or Gazala) requires you to pay some prestige; the amount will depend on your difficulty level. It is quite possible to win the preceding mission but not have enough prestige to choose the ahistorical branch. Also, the ahistorical branch is much, much easier than the historical path, at least on the Eastern Front.