In the early Middle Ages, Switzerland was part of the Frankish empire and later the Holy Roman Empire. In 1291 CE the cantons Schywz, Uri and Unterwalden formed a confederacy, which later expanded further. In the battles of Morgarten in 1315 CE and Sempach 1386 CE they effectively fought themselves to independence. The Swiss set up a militia system that created a pool of well trained warriors who were always ready to defend the cantons. But with war was not too common and the economy poor. So from 1424 CE onwards many used their skills to hire themselves out as mercenaries. Despite offering themselves to the highest bidder, they would never fight against their fellow countrymen. When the Swiss confederacy was in danger, they would abandon their jobs to defend their home country.
As mercenaries, the Swiss fought as they did at home. In their early years many used halberds, but quickly the pike became the favored weapon. The first pikes were about 3 meters long, later extended to 5½ meters. The employed them in dense and deep formations. With their numbers, discipline and ferocity they could break most enemy infantry formations. Cavalry charges were repulsed with the same pikes and any cavalryman who managed to break through was unhorsed and killed by men wielding halberds, guisarmes and lucern hammers. The ideal pike square seems to have numbered 25x25 = 625 men, 75% pikemen, 10% halberdiers and 15% men armed with missile weapons. Despite the need for cohesion as a unit, the Swiss marched quickly, often outmaneuvering their opponents. They marched equally happy in night and day and often surprised enemy garrisons. Much of their aggression was fueled by their poverty. Many Swisss were too poor to afford good armor, so they had to cut down the enemy before the other side took themselves out. They fought very bravely, at times almost suicidally.
When they were victorious, they did not take prisoners but killed captives instead. This, together with a string of victories, made them respected and feared by enemies and desired by employers. They excelled in the Burgundian Wars and were hired by various Italian city states and the kings of France, Spain and England. Gradually they adopted crossbows to gain some firepower, though the pike remained their main weapon.
Around 1500 CE Swiss weapons and tactics were copied by German Landsknechte, who became bitter rivals of the Swiss. The Swiss lost their monopoly. Worse, their tactics were steadily rendered obsolete by two-handed swords and especially gunfire, as illustrated in the Battle of Bicocca in 1522 CE. Despite being reduced from the dominant to just one of the many forces on the battlefield, Swiss mercenaries continued to be valued and serve in foreign armies into the 19th century CE. During the 16th century they remained primarily pikemen; later they adopted other weapons and tactics. Only one unit of Swiss mercenaries survives until this day: the Papal Guard of the Vatican.