After the Battle of Pavia in 1525, in which the German Landsknechte won against Swiss Mercenaries, the Landsknechte mostly replaced the Swiss as the Backbone of 16/17th century Armies. How did the Tactics and Training of these two Mercenaries differ? Why were the Landsknechte preferred after Pavia?

The battle of Pavia is the rise of gunpowder. It is not specifically the defeat of the Swiss, it is also the defeat of the medieval knight.

I'll give some background on military tactics that were evolving and then go over the battle. I will comment some on the Swiss vs Landsknecht question afterwards.

Francis had been going out of his way to prove that the knight was still the dominant force on the battlefield. He would prove this by defeating the Swiss at the battle of Marignano. At that battle Francis would pioneer his new tactics. He knew that the densely packed Swiss phalanx was unstoppable from the front. Francis also knew a cavalry charge was suicide, so he brought a lot of artillery. The artillery would fire en masse and give the Swiss two choices: scatter before the bombardment, or stand in close formation and die. Once the infantry started to scatter the French knights would charge in and obliterate the infantry. Francis went on to use this wrecking ball strategy to great effect in many other battles until Pavia.

So at Pavia the Spanish/Holy Roman Empire(HRE) faced off against the French/Swiss and their allies.

The Battle of Pavia started off as a siege. The French had caught the HRE portion of the army and besieged them at Pavia. Because it was a siege this means the French army was divided up to surround the city, this will prove crucial when the battle happens because the French are spread out.

When the Spanish reinforcements arrived the HRE garrison sallied forth and the Spanish army attacked from another angle. The Spanish hurled 8000 Landsknecht supported by arquebus against 3000 Swiss pikemen. The Swiss managed to hold the line long enough for the rest of the French to form up, but were badly mauled. Another few thousand Swiss were on the other side of the city still maintaining the siege and had no idea what was happening until the garrison sallied out and attacked them.

Francis arrived with his knights and charged immediately to try to break through the Spanish and save the divided swiss. Francis charged before his artillery could soften up the Spanish/HRE landsknecht. His artillery dare not fire because they would be shooting into the back of their own elite cavalry. The cavalry charge was stopped by the Pike and Arquebus tactics of the Spanish/HRE. Francis desperately ordered his own landsknecht infantry to charge in and help him disengage, but Spanish/HRE landsknecht flanked and crushed the French non-landsknecht infantry and then encircled the rest.

The deciding factors in the battle were:

French Artillery couldn't fight because it risked friendly fire on their King.
Spanish landsknecht infantry was supported by arquebus firepower which forced the Swiss to come to them because the Swiss had no guns.
French being badly deployed due to the siege and failing to act decisively to break off the siege and reform.

Landsknecht defeated traditional Swiss tactics because they added firepower to their pike squares, but Pavia isn't a good example of this because at Pavia they just had overwhelming numbers. Landsknecht simply copied Swiss tactics, but adopted the arquebus better than the Swiss. This is also the reason why the Spanish Tercio will go on to defeat both the Swiss Pikes and the Landsknecht. The Tercio would just add even more firearms and a scattering of swordsmen to create a pike block that could shoot.

No successful general after Francis would rely so much one heavy cavalry charge to win the day. Cavalry simply couldn't charge dense infantry blocks unless the Artillery had broken up the infantry formations first. This is as true at Pavia as it is true at Waterloo 300 years later. If you had more guns you forced the enemy to come to you, if you had more artillery you forced your opponent to spread out and lose formation, and the new pike and shot armies were just far cheaper than knightly retinues so they could simply be bigger than medieval armies. The battle of Pavia is about the triumph of gunpowder. It's the day the knight died.