Mas si osare un extraño enemigo

This line shifts from peace and divine destiny back to threat: “But if a foreign enemy should dare…” It introduces a hypothetical invasion or attack. The anthem’s logic is: Mexico is destined for peace, but if someone violates the homeland, Mexico will defend itself.

Mas is a formal or literary way to say “but.” Osare comes from osar, meaning “to dare,” and it is an old-fashioned subjunctive form. Extraño enemigo literally means “strange enemy,” but in this context it means a foreign or outside enemy, not merely an odd person. The line is completed by the next one, which says what the enemy might dare to do.

In the national anthem, the phrase “Mas si osare un extraño enemigo” means “but if a foreign enemy crosses our path”.