What does 2 Samuel 11:1 mean?

In the spring, when kings usually go out to war, David sent Joab and his officers and all Israel’s army to attack the Ammonites. They destroyed the Ammonite land and surrounded the city of Rabbah. But David stayed behind in Jerusalem. 2 Samuel 11:1 - Modern Text Bible

(The next year, David sends his army to continue the fight, but he himself stays home, which is unusual for a king.)