It is probably a reference to the victory of Joshua and company over the kings of Canaan. This includes the conquest of Jericho (Joshua 6:1–27), the fall of Ai (Joshua 8:1–35), and the successful campaigns in the south and north of Canaan (Joshua 10:1 – 11:23). Joshua 12:1–24 lists the kings defeated by Joshua. Joshua 21:43–45 attributes all this “to God and his faithfulness to the promises he had made to Israel. None of it, in the final analysis, was Joshua’s work. All of it, from beginning to end, was a 'great work' that the Lord had done for Israel in fulfillment of his covenant promises (Judges 2:7). To experience that 'great work' was to experience firsthand the mighty power and trustworthy character of God, just as an earlier generation had experienced them in the exodus from Egypt in the time of Moses."1
7 And the people served the LORD all the days of Joshua, and all the days of the elders who outlived Joshua, who had seen all the great work that the LORD had done for Israel.