Bhutan controls several strategic mountain passes through the Himalayas, allowing travel between Tibet and Assam. These passes are also the only way into the kingdom, and, coupled with its centuries-old policies of isolationism, it has been called the Mountain Fortress of the Gods. The heartland of Bhutan has never been successfully invaded; the British, while establishing a protectorate over the nation, did so with threats to the low-lying territories below the highlands.