system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are the most common reasons countries "decide" to become another, and how? E,g. Castile-Aragon to Spain, Prussia to Germany, etc.

***EDIT: Bonus points, explain Austro-Hungary/Spanish Habsburgs?***
gpt
They merge with another country.  

Prussia merged with other German principalities and united as Germany.  Spain was formed through marriage and conquest.  

To unite a new kingdom made of dozens of former nations, they rebranded the country with a new name.