At the advice of Catherine of Sienna, the pope returned to Rome in order to restore the papacy to its former glory. Shortly after, as popes often do, he died. The next pope was elected with the consent of the French, but soon got on their bad side, and lost their support.
So the French elected their own pope in 1378. And for so for around 37 years, there were two popes (actually, there were even three at one point, ironically instated by the Council of Pisa in 1409, which was held in order to resolve the two-pope problem).
Basically, from 1378 to 1415, we had the glorious period of.... Pope Fight!