Characterizing the projective space after Cho, Miyaoka and Shepherd-Barron

In this paper we give a short and elementary proof of a characterization of the projective space in terms of rational curves which was conjectured by Mori and Mukai. A proof was first given in a preprint by K. Cho, Y. Miyaoka and N. Shepherd-Barron. While our proof here is shorter and involves substantial technical simplifications, the essential ideas are taken from that preprint.