Verb (5-dan, ぐ, transitive)
1. to defend against; to protect against
敵を知らなければ敵の攻撃を防げない。If you do not know the enemy, you cannot defend against the enemy's attacks.
2. to prevent; to avert; to avoid
国民の動揺を防ぎたいらしい。It seems they want to prevent public unrest.