I often fall into this dilemma of whether to set the start_date to the treaty date or to the invasion date. Treaty date is often easier when it involves a lot of relations such as the Peace of Paris where the British Empire, French Empire and Spanish Empire swapped multiple territories. Often the treaty is within a year or two of the war, but sometimes it can be 10 years, or 50 years.
I would like some feedback from other mappers.
I’d go for the treaty date (ideally when it came into force rather than when it was signed, if that can be found/is different). Invaded simply implies troops have entered the area.
There’s a bit of a grey area when territory is outright unilaterally annexed though. If something is annexed then that usually means that from the POV of the annexor it is legally considered part of it, so in that case you could set the start_date for the annexor to when they legally declared it annexed and the annexee to when the treaty is effective. Once we get disputed territories etc worked out they can be mapped as disputed from the annexation until the treaty date.
There’s also a middle ground where territory is occupied but not annexed, as well as for occupational administrative entities, puppet states etc but again these should really be their own thing.