The two defensive options against amphibious invasion are strong defense of the beaches and defense in depth. Early in the war (such as at Tarawa), the Japanese employed strong defenses on the beaches with little or no manpower in reserve, but this tactic proved vulnerable to pre-invasion shore bombardment. Later at Peleliu, Iwo Jima, and Okinawa, they switched strategies and dug in their forces in the most defensible terrain.