“Wherefore it is said in the book of the wars of the Lord, What he did in the Red sea, and in the brooks of Arnon,” Numbers 21:14
The Book of the Wars of the Lord is a book referred to in Scripture but now lost to us. The only information we have about the Book of the Wars of the Lord is what is found in one passage of Scripture. The way it is mentioned in Scripture indicates that it was somewhat well-known or accessible to the people of ancient Israel. Such “lost books” are not uncommon, because very few ancient books have survived to the present day. Given the impermanency of most ancient writings, it is a testament to the miracle of Scripture that the books of the Bible have done more than survive—there are so many ancient copies that we can, with great certainty, discern what the originals said.
The biblical passage that mentions the Book of the Wars of the Lord concerns the Israelites’ trek through the wilderness on their way to the Promised Land:
“And the children of Israel set forward, and pitched in Oboth. And they journeyed from Oboth, and pitched at Ijeabarim, in the wilderness which is before Moab, toward the sunrising. From thence they removed, and pitched in the valley of Zared. From thence they removed, and pitched on the other side of Arnon, which is in the wilderness that cometh out of the coasts of the Amorites: for Arnon is the border of Moab, between Moab and the Amorites. Wherefore it is said in the book of the wars of the Lord, What he did in the Red sea, and in the brooks of Arnon, And at the stream of the brooks that goeth down to the dwelling of Ar, and lieth upon the border of Moab.” Numbers 21:10–15
According to Numbers 21, Israel was careful to stay out of Edomite, Moabite, and Amorite territory, camping along the Arnon River. Edom was descended from Esau, and Moab from Lot; these were not part of the Canaanite people that Israel was supposed to destroy. In Numbers 20, Moses had requested permission to pass through the land of Edom, since it was a more direct route to the land of Canaan. Edom refused permission and mustered an army to keep Israel out, but Israel simply withdrew and traveled another direction.
Ancient borders were often a source of contention, and Israel was very careful to stay out of Moabite territory. Moses sent messengers to King Sihon of the Amorites, requesting passage through their territory, promising..
“Let me pass through thy land: we will not turn into the fields, or into the vineyards; we will not drink of the waters of the well: but we will go along by the king's high way, until we be past thy borders.” Numbers 21:22.
However, Sihon refused to allow the Israelites into his land and mustered the Amorite army against Israel. By the power of the Lord, the Israelites defeated the Amorites and camped in their territory (verses 23–26). If the citation of “the poets” in verse 27 is a reference to the authors of the Book of the Wars of the Lord, then another passage from that book is used to poetically relate the overthrow of the Amorite kingdom (verses 27–30).
From the title, “The Book of the Wars of the Lord,” some assume that this was a book about battles in which the Lord fought or perhaps were fought on His behalf; and it may have been just such a book. However, Hebrew books were often named by the first word or phrase in the book and were not a summary of the book’s contents. For instance the book of Exodus is called Shemot in Hebrew, which means “names.” The book of Exodus begins, “These are the names of the sons of Israel who went to Egypt with Jacob.” Thus, the title of an ancient book might tell very little about the content of the book.
In summary, we know little about the Book of the Wars of the Lord, but we may assume that this ancient work documented geographical boundaries between nations and possibly related poetic descriptions of particular battles. It is even possible that the Moabites may have known about the Book of the Wars of the Lord. In any case, the work is cited in Numbers to help substantiate the claim that Israel did not encroach on Moabite territory.