The first king was Saul.
Solomon
King David united the Israelites into a single nation. He was the second king of the United Kingdom of Israel and Judah.
King David and his son, King Solomon built what is modern Israel into a great nation.
King Solomon was king in Israel.
Three ways king David was a great leader: he united Israel to one nation also defeated the enimies and he led the nation to salvation
Israel
David helped to make Israel a mighty empire.
The wars provided impetus for the Israelites to be united, as they faced common enemies. The wars were one of the reasons that the Israelites requested a king (1 Samuel ch.8), which made Israel a united monarchy.
Israel
JeroboamAnswerAccording to the Bible, Jeroboam was the first king of the northern kingdom of Israel, reigning after the split in the United Monarchy in 925 BCE. However, there is a developing scholarly consensus that there was no United Monarchy as described in the Bible, and that Omri was probably the first real king of Israel. Omri ruled from 876 to 869 BCE (another estimate: 884-872).
King Saul.
There is no such thing as 'The United Nation of Israel', however Israel is a member of the UN. Some people refer to the Davidic and Solomonic Kingdom of Israel as the "United Kingdom of Israel" to avoid confusing it for the later and distinct "Kingdom of Israel". Both of these kingdoms occurred between 1000 B.C.E. and 500 B.C.E. and are unrelated politically to the Modern State of Israel. (This is similar to how the Roman Empire is unrelated politically to the Republic of Italy even though they occupy some of the same territories.)
The United States of America