Secede literally means, "to withdraw from an organization." I can't think of a word more apt for the possibility of Turkey's withdrawal from NATO - literally withdrawal from the NATO organization
The word "secede" in this context leapt immediately to mind and then onto the page.
Perhaps there's a UK versus USA nuance. In the USA, "secede" as I've used it is proper, although "withdraw" is also apt.
If there's a nuance between "withdraw" and "secede," to my mind the former is simply the act of removing an entity from a collective - whereas secede seems more serious - with the nuance that a promise or compact was broken. It's subtle though - but that's my impression.
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As an aside, many years ago I wrote a series of articles for an American computer publication. My articles were accepted and then appeared in print - with my name on them.
I was horrified - however! The magazine's "editor" took many of my sentences and butchered them!
I'm not a perfect writer at all- and can recognize and accept improvements to my prose- but the "improvements" made by that editor were nothing short of criminal!