Right after ancient Israel left Egyptian bondage, God instructed them (through Moses and Aaron) to set up their wilderness camp in a special way. Their encampments were to take the shape of a square, with three tribes residing on each side.
The first grouping of tribes in the wilderness encampment would face East and the rising sun (which was the general direction they were traveling in - Numbers 2:3). They would be the first to leave the camp. Next in line to leave would be, in a clockwise fashion, the tribes on the South side, then those on the West and finally those facing north.
The wilderness tents from the tribes of Judah, Issachar and Zebulon were placed together on the eastern side of the tabernacle and were collectively under Judah's banner. Judah, Issachar, and Zebulun were the three youngest sons of Jacob's (Israel's) wife Leah.
Judah was chosen to lead this first set of Israelites out of their wilderness camp not only because it was the most numerous tribe of Jacob, but also because it was prophesied that the scepter of rule would never leave them (Genesis 49:10). Christ, in the New Testament, is called "the Lion of the tribe of Judah" (Revelation 5:5). The number of fighting men on this side of the camp totals 186,400.
The tribal camps of Reuben, Simeon and Gad were placed together on the southern side of the tabernacle and were collectively under Reuben's banner. Reuben, born through Leah, was the oldest son of all Jacob's children, with Simeon being the second oldest. Gad was born from Jacob and Leah's handmaid Zilpah. All together, they totaled 151,450 fighting men.
The Western side of the wilderness encampment contained the tribes of Ephraim, Manasseh and Benjamin. Benjamin was born of Jacob's favorite wife Rachel. Ephraim and Manasseh were the product of Rachel' son Joseph. They were 'adopted' by Jacob as full tribes (see Genesis 48). This group of Israelites contained 108,100 fighting men.
The tribal camps of Dan, Asher and Naphtali, collectively under Dan's banner, were placed together on the northern side of the tabernacle. Dan is the oldest son of Jacob and Bilhah (one of Rachel's slaves). They totaled 157,600 men who could fight and were the last group in the wilderness to leave.
Because God dedicated the tribe of Levi to serving as priests, they are not numbered among those who could fight. The Levites were required to march in the middle of Israel - with two divisions in front and two in back - as they carried the Tent of Meeting (Numbers 2:17, 33).
According to the Bible, the total strength of Israel's wilderness camp army was 603,550 men.