If you mean before the United States was involved, it would have been china, followed by US territory's like Guam, Midway, Guadalcanal, and Okinawa.
Abyssinia
Manchukuo (Manchuria) in China .
Indonesia, Indo-China.
No one really knows!
Desperate to offset their rapidly burgeoning population, and intent on getting hold of many valuable resources, Japan infamously conquered the north-east Chinese region of Manchuria prior to WW2. This could be seen by many historians as the end of the League of Nations, and the true start of the war.
Not only did they obviously want more territory, but the islands they conquered were rich in natural resources. More money, more land, more power.
called them names
The Rhineland
Abyssinia
Manchuria
Japan invaded Manchuria in 1931 - before the start of World War 2.
The Allied Forces were fighting Germany and Japan at the same time. The German offensive happened to start before the Japanese offensive. It was not "an idea".