Why did the Tumen Battle take place?

At that time, the Red Fourth Army planned to open the Beichuan Valley first and enter the Minjiang River area to the west. Welcome the Central Red Army to the north. Tumen is located in the middle of Beichuan Valley, and there is a narrow passage along Tumen River, which is the main passage in the upper reaches of Fujiang River and Minjiang River. If 80,000 soldiers of the Red Fourth Front Army want to enter the Minjiang River Basin from the Fujiang River Basin in the west, they can only pass through here. 1In April, 935, when the Red Fourth Front Army marched westward at the mouth of Jialing River in Xi Du, it fought in Tumen Township, Maoxian County, Aba Prefecture, Sichuan Province, smashing the plan of "blocking Tumen with more than 300,000 people and fully guarding Beichuan Valley", containing 500,000 troops to pursue and intercept, and successfully advancing into Maoxian County to welcome the Central Red Army northward. This struggle with the enemy is called Tumen Campaign.