v1: Acts 13.21 records that Saul's reign was 40 years. This is a round figure, and may include the reign of Ishbosheth (2 Sam 2.10). It was only during the first few years that Saul truly followed the Lord.
v2-4: Jonathan, Saul's son, led part of Saul's army against the Philistines. This conflict had already lasted for many years, and would continue into the reign of David.
v5: As the Israelites gathered together, the Philistines gathered a massive army. It may have been that the Philistines hired soldiers from elsewhere.
v6-7: In response to the threat, many Israelites hid or ran away.
v8-9: Saul saw that the situation was deteriorating; his army was fearful and shrinking; the Philistine threat was therefore growing.
Samuel had commanded a seven day wait (10.8). But Saul became impatient, and offered the burnt offering. This was a clear disobedience to God's command, for only a priest could make an offering. Even when times are difficult, we must trust in God, and obey His word.
v10-12: At the 'right' time, Samuel arrived, and Saul's error was exposed. Samuel's question, "What have you done?" appealed direct to Saul's conscience. In contrast, Eli's question, Why do you do such things?" (2.23) appealed to his sons' intellect.
Saul's answer reflected his personal feelings and experience and human wisdom, and neglected spiritual principles. He responded to the circumstances and not to God.
v13-14: Saul had failed the test of obedience to God's commands given through Samuel. As a result, he lost the kingdom. Saul was not the man after God's own heart, and God would choose another.
Even though Saul had lost the kingdom, he continued in his role as king, and did not stand down, as perhaps he should. David, the new choice, was humble, and did not grasp at the position and authority.
v15: Saul's army, once 3000 strong, with others joining him, was down to 600.
v16-18: The Philistine army was not far away from Saul's. They sent out three raiding parties, each probably bigger than Saul's whole army.
v19-23: Saul's army was not armed, so they were even weaker. The Philistines had subdued the Israelites by the scheme of being their blacksmiths.