Adam Clarke on 1 Samuel 13:1

Saul reigned one year - A great deal of learned labor has been employed and lost on this verse, to reconcile it with propriety and common sense. I shall not recount the meanings put on it. I think this clause belongs to the preceding chapter, either as a part of the whole, or a chronological note added afterwards; as if the writer had said, These things (related in 1 Samuel 12:1-25) took place in the first year of Saul's reign: and then he proceeds in the next place to tell us what took place in the second year, the two most remarkable years of Saul's reign. In the first he is appointed, anointed, and twice confirmed, viz., at Mizpeh and at Gilgal; in the second, Israel is brought into the lowest state of degradation by the Philistines, Saul acts unconstitutionally, and is rejected from being king. These things were worthy of an especial chronological note.

And when he had reigned - This should begin the chapter, and be read thus: "And when Saul had reigned two years over Israel, he chose him three thousand," etc. The Septuagint has left the clause out of the text entirely, and begins the chapter thus: "And Saul chose to himself three thousand men out of the men of Israel."