1 Chronicles 20:1 In the spring, at the time when kings go off to war, Joab led out the armed forces. He laid waste the land of the Ammonites and went to Rabbah and besieged it, but David remained in Jerusalem. Joab attacked Rabbah and left it in ruins. 2 David took the crown from the head of their king[a]—its weight was found to be a talent(b) of gold, and it was set with precious stones—and it was placed on David’s head. He took a great quantity of plunder from the city 3 and brought out the people who were there, consigning them to labor with saws and with iron picks and axes. David did this to all the Ammonite towns. Then David and his entire army returned to Jerusalem.

a. 1 Chronicles 20:2 Or of Milkom, that is, Molek
b. 1 Chronicles 20:2 That is, about 75 pounds or about 34 kilograms

This is from biblestudytools.com.

1. at the time when kings go out to battle--in spring, the usual season in ancient times for entering on a campaign; that is, a year subsequent to the Syrian war.

Joab led forth the power of the army, and wasted the country . . . of Ammon--The former campaign had been disastrous, owing chiefly to the hired auxiliaries of the Ammonites; and as it was necessary, as well as just, that they should be severely chastised for their wanton outrage on the Hebrew ambassadors, Joab ravaged their country and invested their capital, Rabbah. After a protracted siege, Joab took one part of it, the lower town or "city of waters," insulated by the winding course of the Jabbok. Knowing that the fort called "the royal city" would soon fall, he invited the king to come in person, and have the honor of storming it. The knowledge of this fact (mentioned in 2 Samuel 12:26 ) enables us to reconcile the two statements--"David tarried at Jerusalem" ( 1 Chronicles 20:1 ), and "David and all the people returned to Jerusalem" ( 1 Chronicles 20:3 ).

2. David took the crown of their king . . . a talent of gold--equal to one hundred twenty-five pounds. Some think that Malcom, rendered in our version "their king," should be taken as a proper name, Milcom or Molech, the Ammonite idol, which, of course, might bear a heavy weight. But, like many other state crowns of Eastern kings, the crown got at Rabbah was not worn on the head, but suspended by chains of gold above the throne. precious stones--Hebrew, a "stone," or cluster of precious stones, which was set on David's head.

3. cut them with saws, &c.--The Hebrew word, "cut them," is, with the difference of the final letter, the same as that rendered "put them," in the parallel passage of Samuel [ 2 Samuel 12:31 ]; and many consider that putting them to saws, axes, and so forth, means nothing more than that David condemned the inhabitants of Rabbah to hard and penal servitude.

When I first read the verses... I didn't realize how much I was going to learn this morning.

First, battles were fought in the spring because of weather concerns.
Next... I learned that crowns in that time were not worn on the head... but the crown was suspended above the throne.
David's nephew, Joab, went ahead of David to fight the battle... when it was evident that the battle would be won... Joab called uncle David in to take the crown and the glory.
And I found out that David took the men from opposing armies to do hard labor.

The footnotes say the crown weighed 75 pounds. This commentary, however, says it weighed 175 pounds. So I googled "how much does a talent of gold weigh" and the answer is.... 75 pounds. So there's an error in my online commentary. Bummer.

A dull study turned cool.