1 Kings 16:15, 17

Zimri began to rule over Israel in the twenty-seventh year of King Asa’s reign in Judah, but his reign in Tirzah lasted only seven days. The army of Israel was then attacking the Philistine town of Gibbethon.

So Omri led the entire army of Israel up from Gibbethon to attack Tirzah, Israel’s capital.