Following the Cuban crisis, the Francisco Government sent reinforcements to suppress the rebellion and invited European nations to intervene.
The treaty signed by the four countries of England, France, Austria, and Spain had only been established for a few years, defining clear spheres of influence for each nation.
This was not international law, nor was it recognized by all the countries in the world, but backed by England, France, and Austria, the treaty was more persuasive than international law.
Typically, in colonial matters, the powers resorted to force, and only when force could not solve the issues did they turn to the law.
Undoubtedly, now with internal issues within the Kingdom of Spain and lacking the ability to engage in conflict with the United States in the American region, reasoning became the only option.