I think I understand.
I know others will have strategies, this is just mine.
The Allied player will have to make a decision in the “Support A Pro-Allied Coup In Yugoslavia?” (# 104) DE. So, when the Allied player makes their decision I’ll attack Yugoslavia unless the Allied player has selected NO, in that case Yugoslavia will join the Axis.
The Allied AI will always select YES to DE-104, so in this case Yugoslavia becomes Allied and I’ll attack.
To be prepared for the Allied decision I place three Axis land units of good quality and a German HQ in the Balkans just east of Belgrade. When/if I attack Yugoslavia I will have an easy time capturing the Yugoslavian capital and making them surrender.
The next turn, whether Yugoslavia joins or surrenders to the Axis, I redeploy/move the Axis forces near Belgrade into Romania for Barbarossa.
I hope this helps.