re: What would happen if a state declared independence?

That (violence) applies to the countries of the world that don’t hold diplomatic independence votes.

Wars of liberation may have been the main method of creating new states in the 1800s—which is why those who continue to cite the U.S. Civil War are comparing apples to oranges—but that isn’t the case in modern society.

In the past century, the number of countries has steadily expanded—peacefully.

At the time of the First World War, there were about 50 independent states.
By 1970, that figure had doubled.
Now the number has doubled yet again.
Since 1990–statistically speaking–one new nation has arisen per year.