Short answers, pulled from the story.
The Central Plains War was a Chinese civil war fought in 1929 and 1930 between the Nationalist Kuomintang government in Nanjing, led by Chiang Kai-shek, and a coalition of regional military commanders who had broken from his authority. It was the largest armed conflict of the Warlord Era, fought across Henan, Shandong, Anhui, and surrounding provinces of China's heartland.
The war arose from disputes at the National Demilitarization Conference, which opened in Nanjing on the 1st of January 1929. Regional commanders Yan Xishan and Feng Yuxiang accused Chiang Kai-shek of using the demobilization process to centralize military power under his own authority. The Guangxi clique severed relations with Nanjing by March 1929, and a full anti-Chiang coalition had formed by February 1930 with Yan Xishan at its head.
The war directly involved around 300,000 soldiers from the Nanjing government and approximately 700,000 from the anti-Chiang coalition. After Zhang Xueliang's Northeast Army aligned with Nanjing in September 1930, combined forces on both sides exceeded one million.
The coalition was led by Yan Xishan of the Shanxi clique, Feng Yuxiang of the Guominjun, and Li Zongren of the New Guangxi clique. Yan Xishan assumed overall coalition leadership in February 1930, with Feng Yuxiang and Li Zongren aligned behind him. Wang Jingwei also joined the coalition but was expelled from the Kuomintang by the Nanjing government for doing so.
Zhang Xueliang abandoned his neutrality on the 18th of September 1930 and aligned his Northeast Army with Nanjing. The Northeast Army entered the North China Plain through Shanhai Pass and captured Peking within days, forcing the Shanxi Army to withdraw and the Northwest Army to collapse. On the 4th of November 1930, Yan Xishan and Feng Yuxiang announced their resignations from all their positions, ending the war.
The Central Plains War left the Nationalist government financially weakened, hampering its subsequent Encirclement Campaigns against the Chinese Communist Party. Zhang Xueliang's decision to move the Northeast Army south significantly weakened Manchuria's defenses, contributing to the conditions that led to the Mukden Incident and Japanese aggression in the northeast. Regional factions within the Kuomintang retained their networks and rivalries, creating serious problems during the Second Sino-Japanese War and the Chinese Civil War.