According to 1 Chronicles 11:6, how did Joab become David's chief commander?

Answer: First to strike the Jebusites

David promised that whoever first struck down a Jebusite would be chief commander. Joab son of Zeruiah went up first and became chief.

“Now David had said, “Whoever is the first to strike down a Jebusite will become chief commander.” And Joab son of Zeruiah went up first, and he became the chief.”— 1 Chronicles 11:6 (BSB)