While most readers familiar with Spanish history will be familiar with the Carlists, there are also many who are not, or who are relatively unfamiliar with the Carlists. The author will provide a simple explanation of the Spanish Carlists here. Readers who are clear on this period of history can skip this section.
Before Queen Isabella II, Spain followed male succession law, so the Carlists would not have existed.
However, Queen Isabella's father, Ferdinand VII, only had two daughters. To keep the throne within his own family, Ferdinand temporarily amended the succession law, allowing Isabella to inherit.
Amending the succession law was one thing, but Isabella was only three years old, clearly unsuitable to be queen.
This led to dissatisfaction from Prince Carlos, Ferdinand VII's brother, who should have inherited the throne. Coupled with the nobles who originally supported the Salic Law (male inheritance), they united and called Prince Carlos 'Carlos V', which is the origin of the Carlists.
Readers can search for the subsequent Carlist Wars themselves; in any case, they all failed.
In summary, the Carlists were a faction within the Bourbon family fighting for the throne, but at the end of the day, they were still part of the Bourbon family