How much force would it take to break the blade of a medieval-era sword?

No, of course not. If it were so easy, why would anyone bother with the great expense of making swords, when spears and axes were so much cheaper?

Medieval metallurgy was not the equal of modern, of course, but they did understand how to forge and temper steel pretty well. A sword required time and skill far beyond what you put into a horseshoe - and try breaking a horseshoe by banging it against a rock. Your average young man couldn't break a cheap fireplace poker that way. When steel is properly tempered, it is not only strong but somewhat flexible as well - it could flex to some extent and still return to true, and even under great stress it would bend before it would break. Swing a sword against a solid object and what you'll get is a loud CLANG and uncomfortable vibrations ringing through the blade and up your arm.

A sword COULD break of course, but it would mean either a flaw in the forging process (which normally the smith would have figured out before passing it on to his user) or that it had been weakened, perhaps by being subjected to great heat or cold or by the stress of repeated use in combat. There are episodes of swords breaking in medieval literature, but it is generally treated as a surprise or a tragedy for the character and not as one of the routine risks of combat that medieval fighters were prepared for.

Yes, it would be quite possible - for two reasons. Firstly, the medieval-era swords were not the finely crafted weapons used today, nor were their metal-producing skills at the same technical level as they are today. The swords were somewhat brittle and did smash at times - especially when clashing against each other in bouts. And secondly, as a previous responder noted, the force generated by a young man of average strength would be sufficient to break metal.

I highly doubt an average person could break even a medieval era sword, so long as it was made by a decent smith.

The average strength of a human male is 60 foot pounds, so yes a human could break it.