Judges 1:3

‘ And Judah said to Simeon his brother, “Come up with me into my lot, that we may fight against the Canaanites. And in the same way I will go with you into your lot.” So Simeon went with him.'

“And Judah said to Simeon his brother.” The leaders of Judah sought an alliance with Simeon for their task. Their possessions and inheritances lay near each other, and indeed those of Simeon were within the inheritance of the tribe of Judah, so that, as they lived in close familiarity with each other, their interests were closely connected.

“ ‘Come up with me into my lot, that we may fight against the Canaanites. And in the same way I will go with you into your lot.' So Simeon went with him.” The suggestion was that they should join forces, first in securing Judah's allotted territory and then in securing Simeon's allotted territory. And Simeon agreed. The negotiation would take place through the elders of each tribe, the ruling body comprising clan (sub-tribe) leaders and men of experience.

To some extent in the future Simeon would be assimilated into Judah, but they always maintained an independent existence in that union. They provided more men for David than Judah did (1 Chronicles 12:24) and under Hezekiah they won a significant victory against the Amalekites (1 Chronicles 4:41). When Israel split into two kingdoms they appear to have had divided loyalties, some joining the ‘ten tribes' (this may simply mean ‘a number of tribes' in accordance with number usage, compare Genesis 31:7), others remaining with, or later returning to, the house of David (2 Chronicles 15:9).