Secession rarely succeeds.
The realities are cruel. Secessionists are romantic, poorly armed and led. Central governments are realistic, well-armed, and uninterested in losing parts of their geography, resources, population, and so on.
Hence, there are more bloody than “velvet” divorces.
The cogent example for United States citizens is the Civil War. Although current sociopolitical preferences want to emphasize that it was fought to free African-American slaves from bondage, a more powerful motivating force was to “preserve the union.”
In a manner that now is difficult to internalize, a democratic and republican “union” was a revolutionary concept without historical precedent and of relatively recent creation (“four score and seven years ago.”) It was the “union” (“now and forever; one and inseparable.”) that drove the Union forces.
Despite some calls to “let the erring sister go” and the countervailing logic of “states’ rights,” Northern citizens deemed national unity worth shedding blood, and indeed, 750,000 Americans died in the fighting.
Consequently, perhaps traces of memory even a century old prompt little U.S. sympathy for secessionist movements.
And during the past generation, there has been a virtually unending string of minority populations that have sought independence from central governments. In no particular order, these include Ibos in Nigeria; Bosnians, Serbians, Slovaks, and Croats from the shards of Yugoslavia; Papua New Guinea; Sudan/South Sudan; Bangladesh (formerly West Pakistan); Malaysia and Singapore; Sri Lanka, and so on.
And, reviewing global circumstances, there are certainly candidates for division: Belgium in an always unhappy union of Flemish and Walloons; India, comprising 29 states and seven union territories with 22 “scheduled” languages; Russia where hunks of the old USSR have already secured independence; and even China where long distances between outlying provinces and differing languages prompt whispers of separation (not just in Tibet).
But the most proximate cases are three: Canada (Quebec); Iraq (Kurdistan); and Spain (Catalonia). Although spread around the world with no common borders, they share some common themes: longstanding senses of injustice against the central government; coherent borders; solid economics; and linguistic unity. Abstractly, they qualify to be independent countries.
Each time, they failed, and while sovereignty remains the preference of approximately 35-40 percent of the population, it is no longer the driving politico-cultural force of the previous generation.

AHMAD AL-RUBAYE/AFP/Getty Images
The Iraqi central government, however, struck back quickly; seizing major areas of “Kurdistan,” notably the oil-rich Kirkuk. Kurdish resistance was minimal; Barzani resigned; and (once again) the Kurds are far from independence.

Technically, problems appear tractable: a lighter touch by Madrid; more central government funding; greater protection for the Catalan language; and symbolic recognition of Catalonia as a “nation” are among the suggestions.
Fortunately, there has been no bloodshed, so a December election could rally those Catalans preferring continued association with Madrid.
What today’s secessionist movements demonstrate, however, is how difficult it is to implement Woodrow Wilson’s principle of self determination of peoples. And, more often, the cynical corollary, that every little language doesn’t deserve its own state prevails.