It looks to me like you are asking what happened in 1919 that made the outlook for peace worse in 1920 than it had been in 1919. I would argue that the Treaty of Versailles, signed in May of 1919, made the outlook for peace worse (though this is not to say that Germany was not at fault for what was to come). So I would answer your question by saying that the prospects of peace were worse in 1920 than in 1919 because of the Treaty of Versailles.
You can argue that the Treaty of Versailles reduced the likelihood of peace because of how harshly it treated Germany. You can say that the demand for reparations and the taking away of German territory made it inevitable that Germany would be unhappy and would eventually break the peace.
You can also say that the US's failure to sign the Treaty and to join the League of Nations made peace less likely. With one of the most important countries not in the League, that body had much less moral authority to stop countries from acting aggressively.