Although Manchuria is home to the agricultural lands of the Liao River valley, its location beyond the northern mountains relegated it to the relative periphery of Chinese concern. When Chinese state control became weak, at various points in history Manchuria fell under the control of the forest peoples of the area, including the Jurchens and the Manchus. The most crucial route that links Manchuria and the North China Plain is a narrow coastal strip of land, wedged between the Bohai Sea and the Yan Mountains, called the Shanhai Pass (literally the mountain and sea pass). The pass gained much importance during the later dynasties, when the capital was set in Beijing, a mere away. In addition to the Shanhai Pass, a handful of mountain passes also provide access from Manchuria into China through the Yan Mountains, chief among them the Gubeikou and Xifengkou ().