Deuteronomy 2:14 (ESV)

14 And the time from our leaving Kadesh-barnea until we crossed the brook Zered was thirty-eight years, until the entire generation, that is, the men of war, had perished from the camp, as the LORD had sworn to them.

Crossing the brook Zered was a watershed moment for God's people, for thirty-eight years had gone by since Israel left Kadesh-barnea. During those years, the rebellious generation, who the Lord had sworn would not enter the Promised Land, all died. Moses spoke of the death of the men of war as a kind of warning, for the instruction of the current generation.