According to the Hebrew Bible, the Tribe of Reuben (Hebrew: שֵׁבֶט רְאוּבֵן, Shevet Re'uven, Šḗḇeṭ Rəʼûḇēn) was one of the twelve tribes of Israel.
From after the conquest of the land under Joshua until the formation of the first Kingdom of Israel in c. 1050 BC, the Tribe of Reuben was a part of a loose confederation of Israelite tribes. No central government existed, and in times of crisis the people were led by ad hoc leaders known as Judges. (see the Book of Judges) With the growth of the threat from Philistine incursions, the Israelite tribes decided to form a strong centralised monarchy to meet the challenge, and the Tribe of Reuben joined the new kingdom with Saul as the first king. After the death of Saul, all the tribes other than Judah remained loyal to the House of Saul, but after the death of Ish-bosheth, Saul's son and successor to the throne of Israel, the Tribe of Reuben joined the other northern Israelite tribes in making David, who was then the king of Judah, king of a re-united Kingdom of Israel. According to the Book of Chronicles, Adina and thirty Reubenites aided David as members of his mighty warriors in conquering the City of David. However, on the accession of Rehoboam, David's grandson, in c. 930 BC the northern tribes split from the House of David to reform a Kingdom of Israel as the Northern Kingdom. Reuben was a member of the kingdom until the kingdom was conquered by Assyria in c. 723 BC and the population deported.
From that time, the Tribe of Reuben has been counted as one of the Ten Lost Tribes of Israel.
Tribe of Reuben Wikipedia
According to Kenneth Kitchen, following the completion of the conquest of Canaan by the Israelite tribes after about 1200 BC, Joshua allocated the land among the twelve tribes. However, "almost all" scholars have abandoned the idea that Joshua carried out a conquest of Canaan similar to that described in the Book of Joshua.
In the case of the Tribes of Reuben, Gad and Menasheh, Moses allocated land to them on the eastern side of the Jordan River and the Dead Sea. (Joshua 13:15-23) The Tribe of Reuben was allocated the territory immediate east of the Dead Sea, reaching from the Arnon river in the south, and as far north as the Dead Sea stretched, with an eastern border vaguely defined by the land dissolving into desert; the territory included the plain of Madaba.
The exact border between Reuben and the Tribe of Gad, generally considered to have been situated to the north of Reuben, is somewhat inconsistently specified in the Bible, with Dibon and Aroer being part of Gad according to Numbers 32:34, but part of Reuben according to Joshua 13:15-16. On that basis, some have suggested that the territory of Reuben was an enclave in the territory of Gad.
According to the Torah, the tribe consisted of descendants of Reuben, the first son of Jacob, and a son of Leah, from whom it took its name. However, Peake's commentary viewed this as a postdiction, an eponymous metaphor providing an aetiology of the connectedness of the tribe to others in the Israelite confederation.
In the biblical account, Reuben is portrayed as having arrived east of the Jordan after leaving Egypt, but rather than taking land on the west of the Jordan, after conquering it under Joshua, instead took land on the east, as they owned a large number of cattle, and the territory seemed suitable for pasture. Israel Finkelstein et al., however, have claimed that lack of evidence for a systematic conquest or the abrupt appearance of a new culture indicates that the Israelites simply arose as a subculture within Canaanite society. The territory of Reuben encapsulated the territory of the earlier kingdom of Sihon.
According to the ancient Song of Deborah, Reuben declined to take part in the war against Sisera, the people instead idly resting among their flocks as if it were a time of peace, though the decision to do so was taken with a heavy heart. According to the Book of Chronicles, during the reign of King Saul Reuben instigated a war with the Hagarites, and was victorious; in another portion of the same text, Reuben is said to have been assisted in this war by Gad and the eastern half of Manasseh. In the Blessing of Jacob, which some textual scholars date substantially later than these events, the tribe is characterised as fickle - unstable as water, and condemned to dwindle in power and size due to the incest of their progenitor laying with Bilhah.
As part of the Kingdom of Israel, the territory of Reuben was conquered by the Assyrians, and the tribe exiled; the manner of their exile lead to their further history being lost.