In reality until Emperor Augustus starting in 2 AD, Rome was always at war. However, the time between the final overthrow of the Etruscans and Caesar was marked with many wars with other cultures.
The Roman philosophy about defense and battle was simple, destroy the enemy before they can attack. In other words go on the offensive. Rome was a military machine. Roman soldiers were first paid in booty. No, not that type of booty, rather the riches of plundering and stealing from the recently conquered.
The desire to become rich, or rather fight on commision was a major factor on the amount of soldiers the army produced. Place on top of this the iron discipline instilled in the Roman Legions. Soldiers were commonly beaten into discipline. If a man disserted, the unit would suffer along with the dissert when captured.
Captured peoples for the most part did not mind being captured by the Romans. Yes, everyone loves their freedom, but the Romans were not cruel like the Etruscans. Romans let people follow their religion as long as they honored Roman gods. Tribute and taxes were small, all that was demanded of the recently conquered was enough men to enter the army.
Eventually conquered people were given citizenship, but not full citizenship. The were able to participate in forum and listen to senators debate.
The first major set of wars the Romans had to fight were the Punic Wars. The Punic Wars were a serious of wars fought against the Cartheginians. The first Punic War was mosly a naval war. The Romans could not compete with the Carthaginian Navy until they caught a break. An abandoned Carthaginian ship washed up on shore from a storm. The Romans were able to copy their ship, produce in mass quantities; and defeat the Carthagineans.
The city of Carthage was located in northern Africa across the Mediterranean Sea. They had interests in Africa and Europe. The Romans fought them because their interests conflicted. The peace treaty was for the most part unfair because the Romans did not see non-Europeans as equals. A boy was on had and witnessed the signings. His father the king made him swear that one day the Romans would be made to pay for this. The name of the boy was Hannibal.
Hannibal started the second Punic War. The most amazing thing about the Second Punic War was Hannibal's march on Rome. Hannibal started in Spain, traveled across the Alps with an entire caravan of soldiers and war elephants, and around the peninsula we call Italy.
Hannibal was able to hand Rome it's worst defeat of all time. At the Battle of Cannae, Hannibal was able to surround and slaughter over 50,000 Roman soldiers. The Scourge of Rome was eventually defeated by General Scipio. Scipio used the same tactics Hannibal used. He invaded their homeland making Hannibal leave Rome and defend his land.
After Hannibal's defeat by the hands of Scipio, Hannibal went into retirement, or rather, hiding. A huge price was placed on Hannibal's head for his capture, but in the practice of the time, Hannibal committed suicide instead of being humiliated in front of the city of Rome.
During these wars, large or small in stature, many men volunteered for combat. In the early years soldiers were upset because when they came back home, their lands and homes were taken over because they were unable to work. The government saw this as a major problem. Therefore, they made the first professional soldiers. When an army was not marching, they were paid a salarium. We call in a salary. Sometimes soldiers were paid in salt because of its ability to preserve food.