Summary: The conquests of both Moses and Joshua are recounted in the first of these chapters. In the following chapters (13-19), the book of Joshua details the division of the Land of Israel between the tribes.
In depth: The Torah recounts the command to divide up the Land of Israel for each of the tribes. While the actual distribution is described at length in these chapters, there is some discussion regarding the method employed. The Torah appears to give contradictory instructions. Firstly, the area of each portion must correspond proportionally to the size of each tribe (Numbers 26:53-54). Yet in the following verse God commands that the land should only be divided up by lottery (ibid. 55).
Now that Joshua had conquered the land and the division into tribal territories could take place, we see that the lottery is employed for each tribe. The Seforno (d. 1550) comments that even though the land must have been divided proportionally to the size of each tribe, the lottery engendered a feeling in each tribe that their portion was given by God rather than a result of mere population control.
Indeed, the Gemara (Bava Batra 122a) describes the process of the division by lottery. Elazar, the son of Aharon and high priest wore the urim v’tumim, a slip of parchment on which God’s ineffable name was written and placed inside the high priest’s breastplate. This was adorned with stones on which the names of the twelve tribes were etched. He would place each hand into a box; one box contained the names of the tribes and the other, the names of each portion of land. The Gemara describes how Elazar would even predict through Divine inspiration which tribe and portion of land would come up before he had even drawn the lots.
Yet the lottery could be seen as more than merely God’s approval for apportionment of land. This moment was the culmination of four centuries of Jewish history which began with God’s covenant with our forefather Abraham. The lottery provided a covenantal tone in which the people witnessed the transfer of God’s land to His people.