【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.