What were the League of Nations weakness?

Answer 1

America did not join even though it was Woodrow Wilson's idea to set up the an international idea promoting peace, but the United Stated did not want to be pulled into Europe's problems and adopted an isolation policy.

Russia was not allowed to join because it's Communist/Bolshevik policies were feared by Western Europe and America, it also killed it's royal family, the Romanovs. Later wars with Poland did not help Russia's chances of entering.

Germany was not allowed to join in 1919 because it had started World War One, but Stresemann did manage to let Germany join in 1926.

The United States, Russia and Germany were among the great powers and having them not entered in the League really undermined it.

The League had no army, especially not having the United States for military support. The League only had France and Britain who were not willing to support the League financially or with the military because it was still recovering from the First World War. So the League could one verbally warn a country not to enter another or tell the countries in the League not to trade with that country but that country could still trade with non-members of the League like America.