Either subordinating conjunction or a relative pronoun may introduce a subordinate clause.
Explanation:
A clause which provides additional information in a sentence is said to be a subordinating clause or dependent clause. A subordinating clause does not have an independent stand in the sentence. It either modifies or provides a component in the sentence.
Think about it, the civil war was between the north and the south. The south were confederates who supported the act of keeping slaves. The north were against slavery.
