be careful how you define success...

When Saul had taken the kingship over Israel, he fought against all his enemies on every side, against Moab, against the Ammonites, against Edom, against the kings of Zobah, and against the Philistines. Wherever he turned he routed them. And he did valiantly and struck the Amalekites and delivered Israel out of the hands of those who plundered them.
1 Samuel 14:47-48

From a political and military viewpoint Saul was a wildly successful leader. He managed to wage war against every enemy of Israel and to defeat them as they rose against the young kingdom. He built a strong army using every political alliance and popular strategy to build rapport with the people. If anyone started getting a reputation for success in any leadership field, Saul recruited them to his organization (see 1 Samuel 14:52). By this means he achieved credit for every remarkable success in his kingdom.

But Saul neglected the Lord. He had low regard for honoring God in his choices. And that became his downfall. God had chosen him, but Saul forgot that fact in the heady giddiness of success. Power and battle highs kept him from seeing that God's honor should have come first. God would eventually be disappointed with Saul to a point of no return (1 Samuel 15:11).

Saul began a slow and deliberately pain-filled descent into madness as a result. His power went to his head and corrupted his soul. He rejected God altogether and as a result, God moved sovereignly to take the kingship away from Saul (1 Samuel 15:22-23). His success would ultimately not be what Saul would leave for a legacy. He would end up drunk on power, a moody temperamental tyrant who obsessed over maintaining control. He ultimately would die by his own hand in battle, overrun by the enemy and helpless as Israel fell before the foe. He would die the opposite of his pinnacle of success.

Yet, from the tragedy of Saul's demise, God would raise David to be king. Israel would reunite under him to find more success and national security as David expanded the nation's borders. David would do more than Saul ever did. And David would lead Israel to bronze age Middle East dominance as the leader who was a man after God's own heart.