The method of advance of the Roman army to the field of battle varied with the chances of attack from the enemy while the troops were on the march.
 

If the country was known to be safe and there was little reason to suspect a sudden attack or ambush, the army proceeded in one long column. In the vanguard was an ala of auxiliary cavalry with its baggage; then followed the legions, each with its own baggage in rear; while the rear flanks of the column were protected by more auxiliary cavalry. This was the normal formation employed by Caesar, and it is analogous to the order of march adopted by Vespasian in his invasion of Judea, of which Josephus gives the following detailed account.
 

The vanguard was composed of light-armed troops and archers drawn from the auxilia, with a view to frustrating any possible enemy attacks. These were followed by a detachment of Roman legionary infantry and cavalry, which preceded a squad of engineers and road-makers. Behind them came Vespasian himself, with his picked bodyguard of infantry and cavalry, representing the head of the main body, which advanced in the following order. The 120 cavalry, which were attached to each legion, followed by a squad carrying siege apparatus, led the way; then came the legionary general with his cohort commanders and bodyguard immediately in front of the legionary aquila, behind which the trumpeters marched.
 

The legion itself came next, drawn up six deep, with a centurion in the rear to keep discipline and prevent men from falling out, and the main body ended with the baggage of the legion carried by servants and beasts of burden.
 

The rear-guard consisted of mercenary troops, which for safety's sake were followed by some optiones and a picked force of legionary infantry and cavalry. A similar picture is given by Arian in his order of march against the Alani.
 

In this case the vanguard consisted of scouts (exploratores) and two alae and six cohortes equitatae of auxilia.
 

The main body was headed by four auxiliary cohorts, followed by the legionary cavalry, who preceded the aquila of legion XV.
 

Then came the legionary commander with his cohort leaders in front of legion XV, which was followed in a similar order by legion XII marching behind its own aquila. On the flank were cavalry, and the rear-guard consisted of allied troops and baggage animals with an ala of cavalry in the extreme rear.
 

The general purpose, therefore, of the normal order of march was to place the legionary troops in the centre of the column and to protect them with auxiliary infantry and cavalry, to bear the brunt of any possible attack, and give the legion time to deploy.
 
 

When the line of advance lay through difficult country, or when hostile attacks were a probability, it was necessary to adopt an order of march which could be easily converted into an order of battle. The disposition most commonly employed in such circumstances was the agmen quadratum, or hollow square formation. The troops were divided into four columns, which marched parallel to each other, so that, if an attack threatened from any quarter, the agmen could be easily transformed into a triplex acies. This formation was adopted by Germanicus against the Bructeri in 14AD, and the dispositions are described as follows by Tacitus: 'Caesar avidus legiones...quattuor in cuneos dispertit. Pars equitum et auxiliariae cohortes ducebant, mox prima legio, et mediis impedimentis sinistrum latus unetvicensimani, dextrum quintani clausere, vicensima legio terga firmavit, post ceteri sociorum.' By this plan of advance the baggage was placed in the centre of a rectangle bounded by the four legions, and, if an attack threatened from either the front or the flanks, the troops could easily be organised by a simple manoeuvre into three lines of defence. A similar formation resembling the agmen quadratum was adopted by Corbulo in his advance against Artaxata. The three legions of his army advanced in parallel columns, 'recepta inter ordines impedimenta', while a thousand cavalry protected the rear.
 
 

Taken from: Parker, HMD. 1928. The Roman Legions Clarendon Press: Oxford