The League of Nations declarations are available to read online. They do not state more than "the establishment of a Jewish homeland". Nothing about borders nor precise location. Certainly nothing about "both sides of the Jordan River". The borders of that homeland were only specified in UN resolution 181 of 1947.
About 2,000 demonstrators protest 'occupation policy and fascist settlers' in Nazareth (Haaretz)
from the article: It's wrong to suggest Israel was direct result of Holocaust