Know how to tear a country to pieces (I.e., Balkanize it)?

Through immigration in such high numbers that those coming in never have a chance to learn the language, culture and assimilate.

Large pockets of immigrants can be used to start uprisings.

Think identity politics.

Whose specialty is that?

Look at history.

What empire was brought down from within using ethnic minorities during WWI?

Immigration is a very big deal.