Most Japanese expansion was during WWI, when Japan decided to join the Allied powers. They did this so they could conquer the German (colonized) islands in the Pacific Ocean and the German colony of Tsingtao, in China. After WWI was over, they kept all of the land they took in the war - expanding the Japanese empire by a good amount. Like what was said earlier, Japan gained a small bit of land when conquering the Chinese territory of Manchuria in between the wars. This victory was short-lived, though During WWII, Japan continued their Pacific island takeover - coming even closer to the US. When Japan attacked Pearl Harbor, there was a turning point. The American forces stopped Japan from taking anymore Pacific islands, until the US finally defeated Japan in the war, distributing the islands over various territories.