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Royal marriage isn't alliance as such. It's more like a bridge between two countries hoping to live in peace or to cement peace treaties and so. RM's cannot be described as alliance because neither one has any obligations to other. Think of it as CNN back then: when the country goes to war and you manage to get RM, you'll see live war without having to fight on anyones side.

Alliances are alliances. One big happy family. Fight for eachother, help eachother at peacetime (well, that job is mostly yours). There is actually no way to compare RM and alliance. They are just with so different purpose.
 
You might think of it as a treaty of non-agression.