Israel slew seven thousand charioteers and forty thousand footsoldiers from beyond the Euphrates River, which included Shobach/Shopach whose name meant a “spiller of blood.” In Jewish legend, the Aramaean Shobach was renowned for his gigantic size, height, and strength whose presence struck terror into the heart of the beholder. Thereafter, Syrians sued for peace and became vassals of David and did not assist the Ammonites in war again against Israel. With the Syrian threat eliminated, Joab marched his army against Ammon in the springtime. Rabbah, Ammon’s capital city, was destroyed.
After the battle in Syria, and perhaps before the assault on Ammon, David sent Abishai, son of Zeruiah, to the Valley of Salt, where he slew eighteen thousand Syrians. Subsequently David, seemingly incongruently with the narrative, established Israelite garrisons throughout Edom, forcing the Edomites into a vassal kingdom. The parallel accounting in the book of 1 Chronicles identified the eighteen thousand slain in the Salt Valley as Edomites rather than Syrians. Josephus too documented that the battle was waged against the Idumeans.
The book of 2 Samuel is not in error. The Salt Valley battle was part of the Syrian campaign, just as the kings of Zobah of Syria were listed immediately following the Edomites in the war King Saul previously fought. The Salt Valley battle was waged just south of the Dead Sea on the border of Edom and Judah, but generally stated to be in Edom and Seir. Knowing this helps to makes sense as to why David established garrisons throughout all Edom as he did in Syria. Just as Hadarezer’s forces from the east came in support of the Syrians, so did their southern Edomite brethren, invading Judah by the Dead Sea. Psalms 60:1–9 connected the Syrian campaign of Aramzobah and Aramnaharaim with the Salt Valley battle in Edom as the same campaign, noting Zobah was the city of Hadarezer and Aramanaharaim between the two rivers, Tigris and Euphrates.