After the death of Numa, the senate elected Tullus Hostilius, the grandson of the man who had led the Romans against the Sabines. Numa had been a peaceful man, but Hostilius turned out to be even more warlike than the first king of Rome, Romulus. He believed that Rome had grown weak under Numa, and set about waging war whenever he could.
The first war Hostilius fought was against Alba Longa, the original home of Romulus. Also, the peoples of both towns were descendants of Aeneas' Trojans. Because of these close ties, when their two armies met in the field Hostilius and the king of Alba Longa, Mettius, decided avoid unneeded bloodshed. Each army happened to contain a set of triplets. It was decided that these brothers should fight as champions for each side. The Horatii Brothers fought for the Roman side, and against them stood the Curiatii brothers. The home city of the losing side would be destroyed.
In the initial battle, all three of the Curiatii were wounded. Two of the Horatii were killed. The sole remaining Roman champion was left to fight the three Curiatii. He however, was the only one not wounded. Not able to fight three men at once, the remaining Horatius turned and ran. The three Curiatii pursued him. One, who was only slightly wounded, managed to keep up. Another, who was wounded more seriously, lagged behind. The third, seriously wounded, fell far behind.
Now that the brothers were separated, Horatius turned around and dispatched of the fastest brother. Then, he met the second brother and killed him as well. The third brother, seriously wounded, was no match for the healthy Horatius. Declaring that he had killed the first two to avenge the loss of his own brothers, and that he would now kill the third for Rome. Horatius then plunged his sword into the neck of the helpless man.
Alba Longa was now subject to Rome's rule, due to the loss of only three men. The Alban people were understandably upset. They therefore their king Mettius set about regaining their freedom through nefarious means. They encourage the nearby Etruscan town of Veii to attack Rome, hinting that they would betray their supposed allies in battle.
During the ensuing battle, however, Mettius could neither gather the courage to betray the Romans nor come to their aid. Instead, he and his army retreated to a hillside to see which side would win. Hostilius turned the Alban inaction to his advantage and beat the Veientians. After the battle the Albans returned to congratulate Hostilius.
Hostilius was no idiot. To prevent any further disloyalty, he declared the town of Alba Longa no more, and had its entire population transferred to Rome, doubling the city's population. He then declared a terrible punishment for their king, Mettius. His mind had been divided between Rome and Veii, and now his body would likewise be divided. His legs were tied to two four-horsed chariots, and then subsequently his body ripped apart by them.
To make room for the new population, the city was expanded to include the Caelian Hill. The wealthiest Alban families were adopted into the patrician class, and the number of the senate increased to include these new Alban families. In order to house this new, larger, senate, Hostilius built the Curia Hostilia, which served as the home of the senate for hundreds of years.
Under Hostilius, Rome's power had drastically increased through his military actions. Soon, however, a meteor shower struck Rome. Not long after Rome was affected by a plague. Both of these events were blamed upon Hostilius' neglect of the gods in favor of war. He at first did not believe it, but after getting the plague himself, Hostilius changed his mind.
Hostilius dusted off the religious manuals of Numa, and began to once again perform the sacred rites. While performing some secret rites to honor Jupiter, he made an error. An angry Jupiter struck down the king with a thunderbolt in the 32nd year of his reign.
Livy. Ab Urbe Condita.