Ish Bosheth, the son of Saul, ruled the northern tribes for two
years after Saul, while David at the same time ruled Judah and
later became king over all the Israelites.
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After Saul's death, David was crowned king by the people of Judah, but Avner crowned Saul's son Ish Bosheth over the other tribes (2 Samuel 2:4-9).
David's kingdom gradually spread over all twelve Israelite tribes, because:
The people of Judah prevailed in battle (2 Samuel 3:1).
Avner eventually rescinded his support for Ish Bosheth, after Ish Bosheth angered him (Avner), in 2 Samuel ch.3. See also 2 Samuel 4:1.
Other former supporters of Ish Bosheth killed Ish Bosheth (2 Samuel ch.4), leaving David as the only king de facto, which everyone willingly accepted (see 2 Samuel 5:1-3).