Skirmishers are light infantry or cavalry soldiers stationed to act as a vanguard, flank guard, or rearguard, screening a tactical position or a larger body of friendly troops from enemy advances. They are usually deployed in a skirmish line — an irregular open formation much more spread out in depth and breadth than a traditional line formation. Their purpose is to harass the enemy — engaging them in only light or sporadic combat in order to delay their movement, disrupt their attack, or weaken their morale. Skirmishers' open formations and smaller numbers can give them superior mobility over the regular forces, allowing them to fight on more favorable terms, taking advantage of better position or terrain and quickly withdrawing from any threat of superior enemy forces.
Skirmishers can be either regular army units temporarily detached to perform skirmishing, or specialty units specifically armed and trained for such low-level irregular warfare tactics. Light infantry, light cavalry, and irregular units often specialize in skirmishing.
Though often critical in screening the main army from sudden enemy attacks, skirmishers are poor at taking and defending ground from heavy infantry or heavy cavalry. In modern times, following the obsolescence of such heavy troops, all infantry has become indistinguishable from skirmishers, and the term has lost military meaning.
Those acting as skirmishers are said to skirmish. A battle with only light, relatively indecisive combat is often called a skirmish.
In ancient and medieval warfare, skirmishers typically carried bows, javelins, slings, and sometimes light shields. Acting as light infantry with their light arms and minimal armour, they could run ahead of the main battle line, release a volley of arrows, slingshots or javelins, and retreat behind their main battle line before the clash of the opposing main forces. The aims of skirmishing were to disrupt enemy formations by causing casualties before the main battle, and to tempt the opposing infantry into attacking prematurely, throwing their organization into disarray. Skirmishers could also be effectively used to surround opposing soldiers in the absence of friendly cavalry.
Once preliminary skirmishing was over, skirmishers participated in the main battle by shooting into the enemy formation, or joined in melée combat with daggers or short swords. Due to their mobility, skirmishers were also valuable for reconnaissance, especially in wooded or urban areas.
In classical Greece, skirmishers originally had low status. For example, Herodotus, in his account of the Battle of Plataea of 479 BC, mentions that the Spartans fielded 35,000 lightly armed helots to 5,000 hoplites yet there is no mention of them in his account of the fighting. Often Greek historians ignored them altogether, though Xenophon distinguished them explicitly from the statary troops. It was far cheaper to equip oneself as lightly armed as opposed to a fully armed hoplite – indeed it was not uncommon for the lightly armed to go into battle equipped with stones. Hence the low status of skirmishers reflected the low status of the poorer sections of society who made up skirmishers. Additionally, "hit and run" tactics contradicted the Greek ideal of heroism. Plato gives the skirmisher a voice to advocate "flight without shame," but only to denounce it as an inversion of decent values.
Nevertheless, skirmishers chalked up significant victories in this period, such as the Athenian defeat at the hands of the Aetolian javelin men in 426 BC and, in the same war, the Athenian victory of Sphacteria.
Skirmisher infantry would gain more respect in the subsequent years as their usefulness was more widely recognised and as the ancient bias against them waned. Peltasts, light javelin infantry, played a vital role in the Peloponnesian War and well equipped skirmisher troops such as Thureophoroi and Thorakites would be developed to provide a strong mobile force for the Greek and Macedonian armies.
Celts did not, in general, favour ranged weapons. The exceptions tended not to include the use of skirmishers. The Britons used the sling and javelin extensively, but for siege warfare, not skirmishing. Among the Gauls likewise, the bow was employed when defending a fixed position. The Celtic lack of skirmishers cost them dearly during the Gallic Invasion of Greece of 279 BC, where they found themselves helpless in the face of Aetolian skirmishing tactics.
In the Punic Wars, despite the Roman and Carthaginian armies' different organisations, skimishers had the same role in both: to screen the main armies. The Roman legions of this period had a specialised infantry class called Velites that acted as skirmish troops, engaging the enemy before the Roman heavy infantry made contact, while the Carthaginians recruited their skirmishers from native peoples across the Carthaginian Empire.
The Roman army of the late republican and early imperial periods frequently recruited foreign auxiliary troops to act as skirmishers to supplement the citizen Legions.
The medieval skirmishers were generally armed with crossbows or longbows wielded largely by commoners. In the fourteenth century, although long held in disdain by Castilian heavy cavalry manned by the aristocracy, the crossbowmen contributed greatly to the Portuguese victory at the Battle of Aljubarrota. Similarly, English archers played a key role in the English victory over French heavy cavalry at Crécy. In the next century they largely repeated the feat at the Battle of Agincourt. Such disasters have been seen as marking the beginning of the end of the dominance of the medieval cavalry in particular and heavy cavalry in general.
The Seven Years' War and American Revolutionary War were two early conflicts in which the modern rifle began to make a significant contribution to warfare. Despite its lower rate-of-fire, its accuracy at long range offered advantages over the smoothbore musket in common use among regular armies of the time. In both those wars many American frontiersmen served in the militia. The Continental Army during the American Revolutionary War was assisted by such irregular troops, such as the Minutemen. They engaged in skirmishing tactics, firing from cover rather than in the open field engagements customary at that time. Their tactics were influenced by experiences fighting Native Americans. Militia in a skirmish role was particularly effective in the Battle of Cowpens. The character of Natty Bumppo in James Fenimore Cooper's novel The Last of the Mohicans was notably called La Longue Carabine by the French due to his skill with the long rifle common among the Colonials.
During the Napoleonic Wars skirmishers played a key role in battles, attempting to disrupt the main enemy force by firing into their close-packed ranks and by preventing enemy skirmishers from doing the same to friendly troops. Because skirmishers generally fought in open order they could take cover behind trees, houses, towers and similar items, thereby presenting unrewarding targets for small arms and artillery fire. Such tactics often made them vulnerable to cavalry however.
A skirmish force screening the main body of infantry became so important to any army in the field that eventually all the major European powers developed specialised skirmishing infantry. Examples included the German Jäger, French Voltigeurs and British riflemen.
While muskets were the predominant infantry weapon of the late 18th century, the British Army learned firsthand of the importance of rifles in the American Revolutionary War and began experimenting with them shortly thereafter, resulting in the Baker rifle. Although slower to reload and more costly to produce than a musket, they were much more accurate and proved their worth in the Peninsular War. Throughout the conflict, British riflemen were able to selectively target and eliminate the officers and NCOs of French forces from outside of musket range.
In the American theater, American riflemen again contributed to British casualties but now had to contend with revised British light infantry tactics.
A consequence of experiences during these wars was a trend to training line troops to adopt tactics that until then had been used only by skirmishers.
The treatise, New American Tactics, by General John Watts de Peyster advocated making the skirmish line the new line of battle, a revolutionary idea at the time. During the American Civil War, it was common for cavalrymen to dismount and form a skirmish line to delay enemy troops advancing towards an objective (for example, the actions of the Union cavalrymen led by Brig. General John Buford on the first day of the Battle of Gettysburg). Skirmish lines were also used to harass enemy probing missions, hampering the other force from gaining an effective intelligence picture by engaging their scouts and likewise forcing them to deploy.
By the late 19th century, the concept of fighting in formation was on the wane. Heavy infantry had disappeared, and all infantry effectively became skirmishers; the term has become obsolete but as late as World War I continued to be associated with battlefield reconnaissance screens (which are essentially modern skirmish lines). As in the American Civil War, the primary role of the infantry skirmish line was to screen the advance of a parent force and disrupt the enemy's own reconnaissance efforts. With the mechanization of modern warfare, the role of infantry skirmishers was more or less combined with those of light cavalry, as mounted scouts in specialized reconnaissance vehicles took over the responsibility of screening large formations during maneuvers in addition to conducting their own probing actions.
With regards to the role of skirmishers as light infantry, some modern military units still use light and heavily armed units in conjunction. For example, the Soviet Army routinely deployed more lightly armed motorized rifle regiments on the flanks or secondary sectors of a motorized rifle division on the offensive, while the heaviest units backed by the heaviest armour would fight in the division's main effort. The modern Military of the United States has light, rapid deployment Stryker brigade combat teams working with heavy mechanized and armored units, with tracked M2 Bradley infantry fighting vehicles and M1 Abrams tanks forming the primary combat force.
South Africa's military doctrine placed a disproportionate emphasis on the use of highly mobile, light mechanized forces that were able to cover ground swiftly while keeping heavier enemy armoured and infantry formations off balance, only engaging when the conditions were favourable. The lightly armed South African units used tactics such as rapid movement, flank harassment, and confusing the enemy with continuous maneuvering to compensate for their inferiority in firepower when faced with Angolan and Cuban forces during the South African Border War. The innovative use of South African reconnaissance units to throw Angolan tank formations into disarray before luring them into ambushes, effectively deploying them as skirmishers, was also a consistent feature of that conflict.