Final answer:
David engaged in military campaigns against neighboring peoples, including the Amalekites, and established a professional army and bureaucracy in Israel with Jerusalem as the capital.
Step-by-step explanation:
When David left Saul, he sometimes engaged in violent campaigns against neighbouring peoples. For instance, according to 1 Samuel 30:17, David smites the Amalekites, and 2 Samuel 8:1-18 lists some of David's military conquests. Furthermore, 1 Samuel 27:8-11 implies that David left neither man nor woman alive, which suggests that his actions included the killing of children and infants.
David's leadership and military exploits play a significant role in his reputation; his success in battles is often highlighted, as seen in 2 Samuel 8:6, 14, where it is said that "The Lord gave victory to David wherever he went." Moreover, David's rule established him as a key figure in the history of Israel and the founding of a professional army, the development of a bureaucracy, and his capital at Jerusalem.