... by pursuing a policy of imperialism
In the late 19th and early 20th centuries, other nations were pursuing imperialist goals. Japan followed suit. They looked at Korea and China as territories they could control and exploit for their own advancement as a great power. By the end of the 19th century, Japan was in control of most of Korea, and it formally annexed Korea early in the 20th century. Japan also made incursions into Chinese territory during those years, and Japanese invasion of Manchuria in 1931 was a beginning event of World War 2 in Asia.
