The Jewish kingdom began its rise to being a regional power under King Saul in 1020 BCE. His successor, David, raised the status of Israel to that of a major local power using a combination of diplomacy and military successes. It was David who truly united the twelve Israeli tribes into a single kingdom, with its capital city at Jerusalem, when he defeated Ishbaal in 993 BCE. David was followed by Solomon, who ruled—well, okay, I have to say it—wisely until 931 BCE. At that point, Israel was a rich and fairly powerful state tied by treaty with all its neighbors, and it was more than capable of defending itself. Israel under Solomon was a rich trading crossroads; it had developed its copper and other metal industries, and many new cities and towns were founded and old ones fortified. It was under Solomon that the Temple in Jerusalem was built.
The problem arose after Solomon died. To begin with, Israel was prospering, but the Jewish people under Solomon had been subjected to an ever-increasing burden of taxation to pay for defense (including a strong army) and the Temple. So when Solomon died in 931, there was an open rebellion by the ten smaller northern tribes. Under the leadership of Jeroboam, a former court official, they split with the tribes of Judah and Benjamin and founded a kingdom in the northern half of the formerly united Jewish land and established the new capital at Samaria. The two remaining tribes formed the new nation we call Judah, which was ruled by David’s son Rehoboam and whose capital remained Jerusalem; Judah continued to be ruled by the descendants of David.
Somehow during the split, the Jewish people lost the size and prestige needed to stay an important regional power. Less than 200 years later, the Kingdom of Israel was defeated by the Assyrians, and its people were scattered throughout the Assyrian empire, where they soon lost their identity. These are the ten lost tribes. Judah managed to hang on for more than another century before it was overwhelmed by the Babylonians, in 586 BCE. By deciding to split apart, the Jewish tribes may have dealt with immediate problems, but they forever lost the opportunity to become a realm strong enough to survive. Had the kingdom not split, the Jews would likely have been able to maintain themselves as a nation and a people. After all, Judah was able to survive its fall to the Assyrians, and a united Jewish kingdom might have done so as well. Who knows what such a state might have achieved?