Short answers, pulled from the story.
The Second Sino-Japanese War is most commonly dated from the 7th of July 1937, when the Marco Polo Bridge Incident near Beijing escalated into full-scale war, through Japan's formal surrender on the 2nd of September 1945. China's official historiography since 2017 holds that the war began on the 18th of September 1931 with Japan's invasion of Manchuria, making it a fourteen-year conflict.
The war resulted in the deaths of around 20 million people, the majority of them Chinese civilians. Japan's biological warfare attacks alone caused at least 200,000 deaths. The Battle of Wuhan resulted in up to 500,000 Chinese soldiers killed or wounded and up to 200,000 Japanese casualties.
Following the capture of Nanjing in December 1937, Japanese troops perpetrated mass killings and sexual violence against the city's civilian population. The International Military Tribunal for the Far East estimated at least 200,000 murders and 20,000 rapes. More than a third of the buildings in Nanjing and surrounding towns were looted and burned.
Japan used the term "incident" because neither country had made a formal declaration of war, and keeping the conflict nominally limited helped Japan avoid triggering the American Neutrality Acts of the 1930s. The United States was Japan's primary source of petroleum and steel, and a formal war declaration risked an American embargo. Japan only renamed the conflict the "Greater East Asia War" in December 1941 after both sides formally declared war following the attack on Pearl Harbor.
The Three Alls Policy - "Kill all, loot all, burn all" - was implemented from 1941 to 1942 in north and central China, targeting Communist base areas. Japanese forces conducted massacres, mass rape, deportations, poison gas deployment, and weaponized starvation. These campaigns killed millions of Chinese civilians and persisted until March 1945.
China was recognized as one of the Big Four Allied powers during World War II and became one of the five permanent members of the United Nations Security Council. It also regained Taiwan, which Japan had held since 1895 under the Treaty of Shimonoseki. President Franklin D. Roosevelt described the United States, United Kingdom, Soviet Union, and China as the world's "Four Policemen."