How was mendel able to infer that each offspring of two parent pea plants had a pair of "factors" for a particular trait?

The first generation of a cross between true-breeding dwarf or regular height plants produced all plants of regular height. In a cross of second-generation plants, the dwarf trait reappeared. That suggested that the dwarf height factor was there all along but masked in the first generation. So there had to be two factors for the trait.