"If the League had been supported by the US and an army, then the League could have been very effective in preventing wars." How far do you agree with this statement?

The League of Nations had no effective mechanism to prevent war. Articles 10 and 16 of the League Treaty declared members responsible for helping any member under attack to stop the aggressor. It was not clear just how binding these articles were, however, and it was possible for states to interpret them as recommendations rather than obligations.
Nonetheless, under the leadership of Henry Cabot Lodge, the Republican majority in the US Senate decided that these articles infringed on US sovereignty. They argued that, under certain circumstances, the Treaty would allow League members to force the US to participate in an armed conflict against the will of the majority of the US population.
Some Democratic and Republican senators, including Lodge himself, suggested various amendments to the League Treaty. President Woodrow Wilson, however, insisted on accepting the Treaty as a whole without any amendments. In his opinion, if the US introduced any amendments, other countries would follow America’s lead and either force a wholesale renegotiation of the Treaty or accept it with their own additional qualifications.
During the 1930s, the League of Nations failed to stop aggressor states, such as Italy, Japan and Germany, because League members repeatedly demonstrated their unwillingness to risk becoming involved in a major armed conflict in order to implement their Treaty obligations. It is possible that history may have played out differently if they had treated these cases of aggression as criminal acts and applied the rules of internal policing for League members to them. This, however, would have implied that the League of Nations was a coherent political superstructure with the elements of sovereignty, which is precisely what what US public opinion at the time was so unwilling to accept.