This article examines the creation, preservation, and destruction of the defensive forest that the Northern Song built in Hebei along the Song–Liao border. Created as a landscape barrier against the Kitan attacks, this forest established the necessary strategic depth between the capital city and the northern frontline of the Song empire to compensate for Kaifeng's geographical vulnerability. While the Song government painstakingly maintained this forest throughout most of the dynasty, Liao troops, Hebei borderland residents, and many Song officials had nonetheless posed incessant challenges to this military forestation project. In 1122/23, at the onset of the war on the Liao to retrieve the Sixteen Prefectures, the Song army removed this borderland forest that blocked their northern expedition. The destruction of this defensive forest, which could have had thwarted attacks from the north, dismantled the strategic depth between Kaifeng and the Hebei borderland and henceforth presaged the fall of Kaifeng to the Jin, the Liao's successor, in a few years. I argue that this strategic depth was not only a physical distance, but also a diplomatic, sociopolitical, and military link that connected the ecology of the Song's northernmost periphery and the fate of the entire empire.