Although Manchuria officially remained a part of the Republic of China, it became more or less an independent state isolated from China by its geography and protected by the Fengtian Army. The only pass at Shanhaiguan, where the Great Wall meets the sea, could easily be closed. In a time when the central government was barely able to pay the salaries of its civil servants, no more revenues were forwarded to Beijing. In 1922 Zhang took control of the only rail link, the Beijing–Shenyang Railway, north of the Great Wall and also kept tax revenues from this railroad. Only postal and customs revenues continued to be sent to Beijing, because they had been pledged to the victorious foreign powers after the failed Boxer Rebellion of 1900, and Zhang feared their intervention.