God allowed the Ammonites to rise against a section of the nation of Israel in order to solidify Saul's position as the chosen leader in Israel. The Ammonites threatened the inhabitants of Jabeshgilead (an area that had been previously wiped out in the civil war involving mainly the tribe of Benjamin). So, when Saul heard about the threats against the men of Jabeshgilead, he took that proverbial bull by the horns and called passionately for the whole nation to come out in defense of that small group. Clearly his new heart was used by the Spirit of God to cause him to take his leadership responsibilities seriously.
Soon Saul had an army of 330,000 Jewish soldiers to stand in defense of the people of Jabeshgilead. And, Hallelujah, they were victorious. The Ammonites were soundly defeated and scattered. Naturally, on the heels of that victory the people were happily eager to crown Saul as their king and to follow him continually. Up to this point things were going well (in the institution of their new form of government). That would change soon enough though.