Operation Zet was a secret operation of the Soviet Union

Soviet Union

The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....

 to provide military and technical resources to the Republic of China

Republic of China

The Republic of China , commonly known as Taiwan , is a unitary sovereign state located in East Asia. Originally based in mainland China, the Republic of China currently governs the island of Taiwan , which forms over 99% of its current territory, as well as Penghu, Kinmen, Matsu and other minor...

 as a part of the Sino-Soviet Non-Aggression Pact.

The operation was aimed at resisting the invasion of the Imperial Japanese Army

Imperial Japanese Army

-Foundation:During the Meiji Restoration, the military forces loyal to the Emperor were samurai drawn primarily from the loyalist feudal domains of Satsuma and Chōshū...

 during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

. The Soviet Union and The Empire of Japan

Empire of Japan

The Empire of Japan is the name of the state of Japan that existed from the Meiji Restoration on 3 January 1868 to the enactment of the post-World War II Constitution of...

 had a longstanding rivalry that sometimes had resulted in open hostilities.

The operation was carried out in secret to maintain plausible deniability

Plausible deniability

Plausible deniability is, at root, credible ability to deny a fact or allegation, or to deny previous knowledge of a fact. The term most often refers to the denial of blame in chains of command, where upper rungs quarantine the blame to the lower rungs, and the lower rungs are often inaccessible,...

 while simultaneously resisting the expansionist efforts of Japan to establish itself as the pre-eminent political force in East Asia

Greater East Asia Co-Prosperity Sphere

The Greater East Asia Co-Prosperity Sphere was a concept created and promulgated during the Shōwa era by the government and military of the Empire of Japan. It represented the desire to create a self-sufficient "bloc of Asian nations led by the Japanese and free of Western powers"...

. Under the name of Soviet Air Force

Soviet Air Force

The Soviet Air Force, officially known in Russian as Военно-воздушные силы or Voenno-Vozdushnye Sily and often abbreviated VVS was the official designation of one of the air forces of the Soviet Union. The other was the Soviet Air Defence Forces...

 Volunteers, Soviet troops fought in the defense of Nanjing

Battle of Nanjing

The Battle of Nanking began after the fall of Shanghai on October 9, 1937, and ended with the fall of the capital city of Nanking on December 13, 1937 to Japanese troops, a few days after the Republic of China Government had evacuated the city and relocated to Wuhan...

, Wuhan

Battle of Wuhan

The Battle of Wuhan, popularly known to the Chinese as the Defence of Wuhan, and to the Japanese as the Capture of Wuhan, was a large-scale battle of the Second Sino-Japanese War...

, Nanchang and Chongqing

Bombing of Chongqing

The bombing of Chongqing was part of a terror bombing operation conducted by Imperial Japanese Army Air Service and Imperial Japanese Navy Air Service on the Chinese provisional capital of Chongqing, authorized by the Imperial General Headquarters.A conservative estimate places the...

.