In the Warring States Period, why was it so important to join the Party?

If you look at the map, Taihang, Bletilla striata and Hukouji all lead to Shangdang area, among which Hukouguan is more important, which is an important pass between Hebei and Shanxi. From the map alone, if Shanxi wants to expand to the center of North China Plain (Handan or Yecheng), it must pass Shangdang area. As for the troops in North China, there may be more options, but they don't have to go through Shangdang.

Shangdang area mainly refers to today's Changzhi city, which is a highland surrounded by mountains. To its east is the Taihang Mountains, which is the dividing line between Hebei Province and Henan Province. On the west is the junction of Taiyue Mountain Range and Linfen; Wuyunshan and Bafuling border Jinzhong in the north. Danzhuling and Jinquan Mountain in the south are adjacent to Jincheng. Shangdang area has been a strategic place since ancient times.

Extended data:

Shangdang geographical environment

Shangdang, located in the southeast of Shaanxi Province, is the nickname of the four states of Guluze Liaoqin.

Shangdang is a highland surrounded by mountains, bordering Taihang Mountain and North China Plain in the east and Taiyue Mountain and Zhongtiao Mountain in the west.

Shangdang County in Qin Dynasty had a wide range, including Shexian County in Hebei, Yushe in Jinzhong, Zuo Quan, Changzhi and the eastern part of Jincheng (Qinshui in Yangcheng in the west belongs to Hedong County). After the Han Dynasty, the scope gradually narrowed, and it was divided into Leping (now southeast of Jinzhong), Jianxing (now Jincheng) and Yining. In the Sui Dynasty, the remaining Shangdang counties were changed to Luzhou (now Changzhi).

Shangdang has a dangerous terrain and has been a strategic place since ancient times. There are mainly two basins in China, Changzhi Basin and Jincheng Basin, bounded by mountains such as Danzhuling, Yangtoushan and Fajiushan (the traditional boundary mountains of Lu Ze and Lujiang counties).

Baidu encyclopedia-Shangdang