Final answer:
In 1919, the League of Nations was formed with the goal of promoting collective security. The League did not initially include all powerful nations and faced effectiveness issues, leading to its eventual replacement by the United Nations after World War II.
Step-by-step explanation:
In 1919, following the conclusion of World War I, the League of Nations was established by the Treaty of Versailles to promote collective security. The League's goal was to facilitate good relations among countries and prevent future conflicts. However, it did not include all powerful countries initially, with nations like Germany and Russia being left out of its formation. The League of Nations ultimately was seen as ineffective, particularly since the United States Senate refused to ratify the Treaty of Versailles, leading to the US not joining the League. After the outbreak of World War II, which many saw as a failure of the League's purpose, the need for a stronger global organization was recognized, and thus the United Nations was created with broader support, including US membership.
As for the number of countries that joined the League of Nations in 1919, several new nations were established after the war, including Poland, Czechoslovakia, and Yugoslavia. Over time, however, the effectiveness of the League was called into question, particularly as expansionist countries like Japan, Germany, and Italy withdrew from the organization before the onset of World War II.