The story of David in short, as requested:
- Shmuel (Samuel) anoints Shaul (Saul) king of Israel at G-d's command. (Shmuel I 10:1 and ibid. v 24)
- Shaul disobeys G-d, and G-d regrets his anointment. (Shmuel I 15:35 and ibid. 16:1
- Shmuel anoints David as King. (Shmuel I 16:13)
- David serves in Shaul's court (Shmuel I 16:21)
- David and Shaul have a falling out, and David flees.
- Shaul is killed by the Plishtim in battle. (Shmuel I 31:6)
- Ishboshes becomes king, rules over 11 tribes while David rules Yehuda (Judah). (Shmuel II 2:10)
- Ishboshes is killed by his generals. (Shmuel II 4:7)
- David is accepted by all of Israel (Shmuel II 5:3)
We know from the Gemara (Brachos 48b) that "one kingdom cannot overlap another by a hairsbreadth". So when did David become King? When G-d regretted that He had made Shaul king? At David's anointment? When Shaul died? When Ishboshes was killed?