I'm sure this has been discussed before on this forum, it's one of those big "woulda coulda shoulda" moments from the 20th Century.
Alotta factors played into things that made the League ineffective, it wasn't very popular, it was hampered by an overriding desire for pacifism, not to mention it was heavily tied to the Treaty of Versailles, (which emphasised pacifism and disarmament), as well as lacking any real military power, thus the League was reluctant to truly take any real action, nor had any desire to really intervene in situations (like the Spanish Civil War for example). THAT and the fact that a unanimous decision by a certain number of it's members was required, or it wouldn't really act on any resolution.
So... I dunno...
I guess if they hadn't really latched the League onto the Versailles Treaty so much, and MAYBE if the US was more involved (though it too was, at this point, still quite reluctant to involve itself in European affairs), and had been more willing to intervene and take action... they might've made a difference.
*shrugs* But again... I dunno...