Why did Japan surrender in World War 2?

A combination of events such as the atomic bombings of Hiroshima and Nagasaki and the Soviet Unions invasion of Manchuria forced Japan to surrender in World War 2.

Japanese leaders had every intention of prolonging the war as much as they could. The Allies dreaded the prospect of invading the Japanese home islands as the casualties would be high as they would fight tooth and nail for every inch of soil. 

The battle of Okinawa, one of such islands, was the bloodiest for the US army at the time. It only proved that a direct invasion of Japan would lead to catastrophic losses for the allies. Hence the decision to use the atom bombs was taken to get the Japanese to surrender.

For further reading check the following articles:

• World War I
• World War II
• Causes of World War I