Semicolons should be used before coordinating conjunctions—that is, terms such as and, or, and but joining independent clauses—when the clauses are long and have internal punctuation, as shown in the following example:

Since he speaks French, Spanish, and Italian, he assumed he would have no trouble traveling in southern Europe, especially since many people in the region now speak English; but when he arrived in Sicily, his unfamiliarity with the Sicilian dialect made communication difficult.

In the sentence above, the coordinating conjunction but joins the independent clause “Since he speaks . . . English” with the independent clause “when he arrived . . . difficult.” Since both independent clauses contain commas, inserting a semicolon before but makes the sentence easier to read.