From Halopedia, the Halo wiki
An infantry square is a combat formation an infantry unit formed in close order assumes when threatened with cavalry attack.
The formation was developed from an earlier circular formation and was used by the Romans. Plutarch described it as a large infantry square utilized by the Roman legions at the Battle of Carrhae against Parthia, whose armies contained a large proportion of cavalry.
The Han Empire's mounted infantry forces effectively utilized tactics involving highly mobile infantry square formations in conjunction with light cavalry in their many engagements against the primarily cavalry Xiongnu nomad armies during the 1st century CE. Infantry squares were used in the siege of the nomads' mountain settlements near the Gobi region, where Han forces repelled nomad lancer attacks.
The square was revived in the 14th century as the schiltron, and later appeared as the pike square or tercio, and was widely used in the French Revolutionary and Napoleonic Wars.
In 2552, during the Battle of Installation 04, Major Antonio Silva ordered his battalion of Orbital Drop Shock Troopers to form an infantry square to hold out against the incoming Covenant Ghosts pressing down on them until Lieutenant McKay could ready the assault on Alpha Base. The tactic, although described as "archaic," had worked out very well, and kept the casualty ratio relatively low on the Marines.