Technical military literature before the 19th century is always worth reading, and one of the technical writers I often return to is Sir John Smythe who died at Little Badow in England in 1607. Smythe was a diletante and a crank who believed that the military art had been perfected on the day he turned 18, but he followed the wars and had thoughts on different ways of doing things. One of the things he talks about is militia recruits who are left-handed. The history of left-handedness is kind of like the history of queerness, in that some societies loved to talk and theorize about it, while left-handers (or queer people) got on with their lives, found solutions that worked for them, and did not leave many traces or worry too much about those talkers and theorists.
In the first passage, Sir John speaks about the captains who make the pikemen towards the left side of a column carry their pikes on their left shoulders, “to the intent (as they say) that the piquers of the left flanke or side of the band or squadron carrying their piques vpon their left shoulders, doo in the same carrying of their piques, greatly beautifie the band or squadron in the eies of the beholders.” Sir John does not approve, and in explaining why he talks about soldiers who are naturally left-handed as opposed to those who are asked to fight left-handed:
Also if there were any piquers that were left handed from their youth, I wold wish them to carrie their piques vpon their right shoulders, and to practise and vse their piques with their right hands, in couching and making head with them against either horsmen or footmen, and in all other militarie exercises and actions. And because such left handed soldiors doo weare their swordes vpon their right sides, I would wish that they should not be placed neither in single bandes, nor in squadrons, vpon the vttermost flankes or sides of them, but in some other of the inner ranks.https://quod.lib.umich.edu/e/eebo/A12568.0001.001/1:5?rgn=div1;view=fulltext
Most people before the 20th century who talk about fighting left-handed talk about the advantage of training to fight with either hand for agility and muscle development. Fighting with the other hand can also be useful if one hand is injured. Today some shooters find it useful to be able to use a long gun from either shoulder in case they need to shoot from different sides of cover, and American police sometimes practice operating a firearm with their dominant hand disabled. The doctor Petro Fagarola and the fencing master Antonio Manciolino both recommend practicing with both hands to improve strength and nimbleness, and I believe there is something about slinging with either hand in the book of Numbers. Others, like the fencing master Fiore dei Liberi, just mention that some people use their sword in the left hand and do not say why. Smythe is explicit that some soldiers prefer their left hand, and that they should only switch if its necessary to keep order in a dense formation of men with long pikes swaying in the wind.
People are also interested in whether soldiers trained as groups. Smythe has a lot to say about training soldiers to march and skirmish. He has this to say about training them to fight:
Also I would that such soldiors as are piquers being disarmed should with blunt piques, learne to march soldiorly, as also with dexeritie to handle and manage them, thereby the better to know how in seruice to handle their sharpe piques; as also the halbarders with blunt halbardes made for the purpose, should learne to giue both blow and thrusts at their enemies with their greatest aduauntage: Howbeit as for halbarders and piquers there is no neede that they should be so often exercised to the vse of their weapons as the soldiors with weapons of volee; because that soldiors with weapons of volee, be they horsemen or footmen shal neuer be able effectually to performe the vse and effects of their weapons in seruices of the field in earnest, if they be not often and long before exercised in sporte with dexteritie to mannage and vse those weapons. And so likewise vpon such aforesaid holydaies, I would wish that all sorts of horsemen of what weapon soeuer, should either before their maisters or some other principall men where they dwel, appointed for that purpose, ride their horses, and exercise themselues on horbacke, with such weapons as they are inrolled vnto. And all these aforesaid priuate exercises vpon holidaies I would wish to be performed to the intent to make all the soldiors of all sorts of armors and weapons as well on horsback as on foote to be the more apte, & ready to performe the effects of soldiors vpon all important emploiments and seruices Militarie, as also at such generall musters, and trainings as should be appointed, & thought requisite vpon some two speciall times in the sommer, when the people might haue best leisure to be absent from their more needefull and necessarie Countrie affaires.https://quod.lib.umich.edu/e/eebo/A12568.0001.001/1:6?rgn=div1;view=fulltext
Xenophon agreed with this: he wanted horsemen to practice riding over all ground and throwing their javelins as often as possible, but in Cyropaedia he sees teaching peasants to fight in a line as about giving them weapons, teaching them drill, and maybe gathering them in gangs and letting them beat each other up with canes and clods of turf so they get used to striking and being struck.
If you are interested in any aspect of low-tech warfare, Sir John Smythe has something worth reading. Texts in marked-up HTML are free online.
(scheduled 4 November 2022)
Edit 2022-12-06: added picture at head of post. It is 1 Chronicles 12:2 which says that some “mighty men” who helped David could shoot arrows or sling stones with either the right hand or the left
Edit 2022-12-26: see also Ehud in Judges 3:12–30 and 700 slingers of Benjamin at Judges 20:16 (thanks Biblical Archaeology Society)