This line means roughly, “May your temples be crowned with olive, oh Fatherland.” The speaker addresses Mexico as Patria, a feminine personification of the homeland. Instead of talking about a person, the line imagines the country as a noble figure whose head can be crowned.

Ciña is a poetic subjunctive form from ceñir, meaning “to encircle,” “to gird,” or “to crown.” Sienes are the temples, the sides of the head. Oliva means olive, and an olive branch is a traditional symbol of peace. So even though the chorus is full of war imagery, this stanza begins by imagining peace crowning the homeland.