Defend, protect, shield, guard, safeguard mean to keep secure from danger or against attack.
Defend implies the use of means to ward off something that actually threatens or to repel something that actually attacks.
Protect implies the use of a covering as a bar to the admission or impact of what may injure or destroy.
Shield differs from protect especially in its suggestion of a protective intervention comparable to a medieval warrior’s shield before one exposed to imminent danger or actual attack.
Guard implies a standing watch at or over for the sake of defense; it usually connotes vigilance.
Safeguard, much more strongly than any of the preceding words, implies use of protective measures where merely potential danger exists.