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There are protections against separation without good cause. So, the military couldn't just decide to engage in a reduction in force and cause you to be separated.
However, if you were being separated involuntarily, if there is a legal basis to allow separation (like PT failure or misconduct, just as an example), you could be processed for separation.
You'd then have two other protections. First, you'd be entitled to a board hearing. You can present evidence, have an attorney and you can argue for retention with that board. Second, any separation ordered for someone with lengthy service is automatically reviewed at the Secretary level for your branch of service. Often, people are permitted to retire anyway, based on that review.
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