Original sentence: American economic and diplomatic pressure and internal political issues, more than the military success of the Red Army, forced Japan's withdrawal.Japan, which sent by far the most numerous organized force, had its own complex and poorly articulated reasons for participating in the intervention.The Japanese government's intense hostility to communism as a potential threat to its monarchy, a determination to recoup historical losses to Russia following the Treaty of Portsmouth, and the perceived opportunity to settle the “northern problem” in Japan's security by either creating a buffer state, or through outright territorial acquisition were also factors.TheEmpire by then had annexed Korea, Inner Manchuria, parts of China, and had a very strong interest in expanding its influence and territories in the Far East
Edited sentence: However, with the rise of Japanese expansionism, the Empire by then has effectively annexed Korea, Inner Manchuria, parts of China, and had a very strong interest in expanding its influence and territories in the Far East.Japan took advantage of the disorder following the Russian Revolution to occupy Outer Manchuria (Russian Far East), but Soviet successes and American economic pressure forced Japanese withdrawal.Japan, which sent by far the most numerous organized force, had its own reasons for participating in the intervention.Located thousands of miles away from the main battlefields of the war, it could not have made any impact on the potential Eastern front