The breakup of the Soviet Union?

@Ben that one's a wee bit complicated. Wasn't so much a secession as a regime change. The various SSRs that formed the Soviet Union were also technically autonomous states in alliance. You could also argue that the subsequent combat in Chechnya, Georgia, and Ukraine points to the fact that secession is usually not bloodless (the collapse of Yugoslavia is another modem example).

Singapore leaving (kicked out of) the Federation of Malaysia is another example and it was bloodless

@slebetman there's a reason my answer says "almost all"

USSR just collapsed on its own. Each of the "republic" had its own parliament, president and government. Maintaining one central government on top of that was too much for the declining economy. So it was dissolved by a legal agreement of 3 major republics, including Russia.

The answer is ok, but lacking any evidence/quote explaining it in-universe.

Brexit anyone??

@Seamusthedog Brexit is about breaking a set of alliances -- that's all the EU really is.

@yshavit not going to get dragged into anything but just to let you know the EU is much more than 'a set of alliances'

@Seamusthedog the EU is also somewhat less than a federated nation-state, despite the worst rhetoric of the Brexiteers.

Re: "the Senate is described as the 'government', which implies the Republic to be a single nation-state": You mean "state" rather than "nation-state".

Perhaps not as famous as the American Civil War, at least to Americans, is the 2017–18 Spanish constitutional crisis, as an example of a not-so-long-ago crises of a Government that people might have thought to be democratic, hence, not expect to react in this way to separatists.

Perhaps a better example than "Brexit" would be Spain and Catalonia..‌​.

I think we're forgetting one of the biggest examples of secession of all: The American Revolutionary War. Granted, the Separatists in the SW Universe were mainly driven by the Sith and their evil intentions, but the technical points still remain the same. A group of planets grew tired of the Republic's way of doing things and decided they wanted to be independent. So they went to battle to try and win that independence. The British Colonies grew tired of the tyrannical taxes and oppression of the British Crown, so they went to arms and declared themselves independent, then to war.

The difference being, of course, America won their independence. The Separatists did not, and ended up losing their droid army and their ultimate betrayal by the Sith who were using the clone army the whole time as their ticket to establishing the Empire later.

This is more fit for politics or history, but wars for colonial independence are a type of secession, but a bit different. Typically, colonies have no representation in the colonial powers government (although, a typical colonist in Philadelphia in 1770 had the same representation as a worker in Lancashire - ie, none).

I drew the parallel between the US Civil War and the Clone Wars because the secessionist did have representation, but did not like the majority rule