From the standpoint of the US, the CSA was "illegal," but other nations are not held to that judgement.
De Facto
"In fact, or in effect, whether by right or not."
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International Court of Justice Glossary:
De facto - in fact, or in effect, whether by right or not. Anything considered "de facto" should be treated as truth or law, not to be argued against. For example, if a country has been divided "de facto" then that means that are officially considered two nations.