Depends on the time reference and point of view. During the war it seems it was legal. After the war it was made illegal.

During the war the Union honored Confederate paroles, and gave Confederate soldiers prisoner of war status. The Union government and Confederate government participated in a cotton trade with each other, trading cotton for cash. The Union government recognized the Confederate government officials in letters and messages and during negotiations. Confederate agents and commissioners were recognized and received in the houses of government in France, Spain, England and many other European countries. In the Confederacy, European government agents and commissioners were received and accepted in many cities in the Confederacy.

The Southern people accepted the Confederate Government as legit and particapted in the paying of taxes, voting, patents, banking, oaths and all other things associated with a legitimate government.

The Confederate soldier is recognized as an legal veteran by the United States Veterans Affairs.
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David Upton