Tullus Hostilius was the third of the Roman kings.
This warloving king almost immediately sought war with Alba longa. There were always cattle thefts in both directions, so it was easy to demand retribution and send the fetiales to start a war, when denied.
War with Alba Longa
The Alban king Gaius Cluilius, when challenged, marched to Rome and built a camp not more than 10 miles from the city. There he died and the Albans appointed Mettius Fufetius as dictator. He showed more wisdom and proposed another way of dissolving this dispute, because it would be unwise to weaken themselves when the Etruscans, the other neighbors were so much stronger and could take advantage.
The 2 triplets
Very convenient there happened to be in both armies three brothers of the same birth (Horatii and Curiatii). The king and that, whoever of these brothers would win, would bring total victory and the loosing party would submit to the other. Of course the Romans won though only one of all brothers, Publius, survived. see Liv.(1.25)
He marched at the head of the army back to the city. His sister, who happened to be engaged to one of the other brothers, angered him by weening about her fiance instead of mourning about her brothers and he killed her.
Trial and appeal
Should the young man who brought victory to Rome, now be scourged and hanged? Tullus Hostilius dared not take a decision and appointed a duumvir (two-men).
They followed the law and answered the question affirmative. The young man appealed to the people of Rome. His father (also the murdered girl's father) pleaded on his behalf and asked not to render him childless.
The people made the young man bring an extra sacrifice to the gods and the offender had to pass under a low hanging beam (yoke), but kept his life. This sister's beam was still in place in the time of T. Livius
Betrayal and destruction of Alba Longa
The Alban people were not happy with the outcome of this "war" and made this known to Mettius Fufetius. Then another war broke out. This time it was Veii a city nearby Rome, that marched its army towards Rome. Rome marched out together with the Albans.
When the battle started the Albans marched away to see from a distance which army would win. Tullus Hostilus made his army believe he had sent the Albans there to attack the enemy from behind, vowed 12 new salii, temples to Palles and Pavor and won the battle.
After the battle he had Mettius Fufetius torn in two between two chariots and had the people of Alba longa transplanted to Rome and that city destroyed. The city was enlarged and the senate extended with Alban nobles.
Senate enlarged with Alban nobles
He nominated Alban nobles to the senate that this order of the State might also be augmented. Amongst them were the Tullii, the Servilii, the Quinctii, the Geganii, the Curiatii, and the Cloelii. To provide a consecrated building for the increased number of senators he built the senate-house, which down to the time of our fathers went by the name of the Curia Hostilia.
After this he defeated the Sabines in a battle.