Imperial Japan was the regime that led Japan during the dark chapters of World War II. The most infamous period of this regime was during the Showa Era, which is when the Imperial Japanese brutally invaded China and other territories of Asia and the Pacific, and continued to hold these territories until their sound defeat by the Allied Powers.
They were considered to be especially vicious towards conquered civilians as well as their prisoners of war - stemming from a fanatical belief in Imperial Japan that to be captured by one's enemies was a disgrace and thus prisoners were to be treated as cruelly as possible. They claimed to be liberating Asia from previous imperialists, though their actual intentions were to create their own Japanese-dominated empire in Asia.
Imperial Japan were also infamous for their fanatical willingness to die and their reliance on fierce ambush tactics, such as in the raid that devastated Pearl Harbor. They were responsible for massive destruction and the brutal murdering of tens of millions across East Asia during the Second World War, such as in the horrific Nanking Massacre. They also showed an incredibly racist attitude towards non-Japanese Asians, which served as the primary reason behind why they killed so many of them. They were also responsible for the origin of the famous "kamikaze" pilots.
They were also supportive allies with Nazi Germany during World War II. However, their cooperation was limited due to their extensively distant geographical locations.