From approximately 3100 BCE until around 1075 BCE, ancient Egypt was ruled by 20 different dynasties. The length of the dynasties varied: some, such as those during the First and Second Intermediate periods could be quite short, while the 13th and 18th Dynasties each contained more a one dozen kings and ruled over the Nile Valley for around 200 years each. Although the first 20 Egyptian dynasties varied in number of rulers and length, most shared one important attribute: they were all native Egyptian dynasties. The one important exception came during Egypt's Second Intermediate Period when a mysterious foreign group of people, known as the Hyksos, conquered Egypt and established the 15th and 16th Dynasties some time shortly after 1700 BCE. For centuries, the Hyksos rule over Egypt was an enigma shrouded in half-truths and myth. The Hyksos were sometimes mistakenly associated with the biblical Israelites, but were for the most part forgotten in modern times due to the dearth of written texts that can be dated to their rule. It was only in the mid-20th century that Egyptologists, using newly discovered and translated texts, shed fresh light on the Hyksos to reveal details about their origins and rule in Egypt.