Stirrups had been invented before the Mongol invasion, but they were not in widespread use in Europe.

1. Europe was at the ass-end of the Mongol conquest, not one of the main theatres.

2. The stirrup was absolutely in use in Europe by the time of the Mongol Invasion, and had been widespread for some 400 years at that point.

The other reason that the Mongols became less invincible, of course, is that their enemies learned to use their tactics and equipment.

3. They didn’t, though. The Mongols’ tactics and equipment were by no means new.