Become Guatemala is a decision available to United States of Central America. One needs to be in control of all the Guatemalan cores and have lost control of all provinces Guatemalan cores. Note that USCA does not need to own all Guatemalan cores, it needs to own only Guatemalan cores. So British-owned Belize is not required.
In order to take the decision, USCA actually have to become a lot weaker. Therefore, it is from a purely strategic standpoint the worst decision in the game, but it can serve roleplay purposes. In the situation, that USCA has lost all provinces but Guatemalan cores, it does however makes sense to take, as it gives 30 prestige, which is quite a lot. On top of that, Guatemala has Mayan as an accepted culture, while USCA does not.