The noun secession refers to a big break-up — a formal split, an official “Good-bye to you!” — among political entities. If France has really, truly had it with the European Union, then a French secession movement may be in order.
You could think of secession as a kind of divorce for governments, an official and often lasting split between an alliance, federation, or other political group. Secession is the noun version of the verb secede (meaning to withdraw from an organization), and when a secession takes place the group doing the seceding makes a formal departure from the original group. A famous example of political secession happened right before the American Civil War, when eleven southern states withdrew from the U.S. government over the issue of slavery.