What does 2 Samuel 11:18 mean?

A few days or weeks earlier, David had sent a message to Joab, his military commander, who was fighting the Ammonites at the fortified city of Rabbah. David ordered Joab to send Uriah, one of Israel's thirty greatest warriors, back to Jerusalem with news of the battle. Usually, such tasks were handled by messenger, but Joab complied (2 Samuel 11:6). Rabbah, modern-day Amman, capital of Jordan, was several day's journey away and required covering difficult terrain, a river crossing, and significant elevation gain in both directions.

David proceeded to concoct a plan to ensure Uriah went home and made love to his wife. David needed to hide the fact that he had slept with Uriah's wife and conceived a child with her. But Uriah refused to go home while his fellow soldiers were encamped in tents in a field. Now David is resorting to murder. He sent Uriah back with instructions to Joab to make sure Uriah dies (2 Samuel 11:2–15).

David sits and waits. No news can travel faster than a man can ride a horse during this era. David had to wait for Uriah to make the trip, for Joab to conduct his orders to have Uriah killed in battle, and then for a messenger to make the trip back with the news. It's possible that several weeks went by as Bathsheba's pregnancy advances, risking exposure, and David's anxiety soars.

The messenger finally arrives and tells David that Uriah, as well as several others, have been killed. Instead of wrath, David responds with encouragement. Joab is to regroup and take out the city (2 Samuel 11:22–25).

Bathsheba mourns her husband, and David marries her (2 Samuel 11:26–27).