The Second Sino-Japanese War (SJW) was historically started by Japan under pretense of the Marco Polo bridge incident on the 7th of July 1937, which led to 8 years of war in China until Japan's capitulation on September 9th, 1945.
In Hearts of Iron 4, the SJW is represented by a number of mechanics designed to allow for the slow paced warfare that historically took place. This is a feature of the 
Waking the Tiger DLC.
To start the SJW, due to Japan's reluctance to justify war goals under Emperor Hirohito, Japan needs to take the Marco Polo Bridge Incident national focus. Alternatively, Japan can wait until 
world tension hits the 75% mark to justify a war goal or until any Chinese faction nation takes the Pick a Fight With Japan or Provoke Japan national focus and starts a war. In the latter two cases, Japan will bypass the Marco Polo Bridge Incident national focus and enter the conflict without the according debuffs.
National Spirits in the SJW
Japan - Marco Polo Bridge Incident
This Japanese national focus serves to debuff Japan's army in the beginning of the SJW. The debuffs can be undone in five steps by repeatedly taking the escalate the war in China decision. Taking the Secure China focus removes the Marco Polo debuffs entirely and allows the Ichi Go decision to be taken.
Nationalist China and Warlords - Army Corruption
- Division Attack: −50%
- Division Defense: −50%
- Mobilization speed: −30%
The Chinese starting national spirit gives the chinese factions in the SJW debuffs similar, but worse than the Japanese. It can be removed by taking the Army reform decisions.
Japanese Decisions
Escalate the War in China
The decision to Escalate the war in China costs 25
and can be repeated every 30 days. After 5 times, the effects of the Marco Polo Bridge Incident national spirit are gone.
一号作战
The Ichi Go decision gives a buff to Japanese forces vs Chinese factions for the 365 days duration of the Conquer China mission, which on succesful completion gives +5% 
and +10% 
, or the reverse effects if failed. The mission is accomplished by capitulating 
中华民国. It costs 50 PP to take it.
Nationalist China and Warlords Decisions
Army Reform
- Change national spirit Army Corruption:
Army Experience Cost: 100- Division Attack: +16,7%
- Division Defense: +16,7%
- Mobilization Speed: +10%
The decision requires the Army Reform national focus, takes 90 days to complete and costs 100 army XP. Taking it three times removes the army related debuffs for China.
Scorched Earth Tactics
- Damage the
Infrastructure of any states adjacent to occupied states by 3 levels at the cost of 25 Political Power - Re-enactable once every 30 days
Industrial Evacuations
- Transfer
Civilian and 
Military Factories from one state to another within 15 days at the cost of 25 Political Power
Border Clashes
During the SJW, China, Chinese warlord factions and Communist China may engage in 
Border Clashes against each other.