I have a question I don't yet understand. What controls who takes occupancy of a province? Playing as Australia, noted Turkey joined on Axis so thought I'd try for Gallipoli achievement. I did a naval invasion of Italian Rhodes, and instead of me getting it, it went to Greek occupancy? Now every Turkish territory I push into is turning blue for Greece not Green for Australia? I've taken Gallipoli on my own rushing it with motorized, but not achievement as it's Greek occupied.
Only solution I can think of is to clear out my troops and hope Turkey beats Greece and then try again?
You can request the territory from Greece. The AI will generally grant occupancy as long as you're contributing to the war. I'm not sure whether this will give you the achievement though, as I'm not sure how it's triggered.