1. The country (the USA) was involved in a civil war.
2. The country (the USA) was involved in the Civil War.
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What is the difference between 'a civil war' and ' the Civil War'?
"a civil war" is a rather generic term. Many countries have experienced civil wars, and the US is one of them.
The USA was involved in the Civil War, between 1860 and 1864. (This time the words are capitalized because they refer to a particular country's civil war at a particular time.)