In the 5th and 4th centuries BC, migrating Germanic tribes forced the Gallic Celts to push south in search of new territory.  They crossed the Alps and settled in Eturian lands by force.  The Gallic tribes, by customs, maintained their own Kings and Warlords.
An aggressive Gallic Tribe was the Senones under the command of Bennus who led his Celts to Clusunim, 100 miles. south of Rome.  The Etruscans weaken by earlier attacks called to Rome for help.  The Romans sensing an opportunity to absorb the Etruscans offered their support.  Negotiations escalated with the Celts and King Bennus offended, in turn, began an attack on Rome defeating its outnumber defenders who scattered in panic to adjoining towns.  The shattered remnants retreated to the Capitoline Hill hoping to endure the expected siege.  There was considerable agitation to abandon the Hill and flee.  Several uphill attacks on the small, fortified capital were successfully driven back.  The Gauls, in the interim, poured through the city slaughtering the civilians while looting and burning everything in their path. 
Finally, the Romans were able to engage Bennus in serious negotiation and he was finally dissuaded from the battle.  Accepting 1000 pounds of gold for his weakened and sickened (the plague) army, he abandoned the effort and his nomadic tribe moved-on. 
As a consequence of this first and major, vicious tribal, military experience, the Romans adopted a new array of military weaponry, abandoned the Greek Phalanx of long spears in favor of the more flexible Gladius sword and the wearing of Infantry armor.  For the first time, the vaunted Roman Legions were reorganized from top to bottom.  These revisions included the sizes of the new units, leadership at several levels, and the objectives of each unit.  In particular, normally, the patricians (elite) filled the front ranks; in the new formations, the younger, stronger and especially, the braver were located in the front and sensitive ranks.  (click on Roman Legions Hyperlink)