During 1920 and 1935, collective security failed to keep peace because the League of Nation's military power was ambiguous and essentially nonexistent, the major powers of the world were absent from the League of Nations, and the members of the League of Nations deliberately undermined the principles of collective security multiple times. The League did not have its own armed forces nor did the Covenant outline any plans for how these armed forces should be used, which created an ineffective military power. The absence of major powers from the League also signified the absence of major social, economic, and political power. Between 1920 and 1935, there were several instances - including the Ruhr Crisis, the Manchurian Crisis, and the Abyssinian Crisis - where members of the League undermined the Covenant. However, the actual possibility of the League of Nations in being able to maintain peace was already rendered to a minimal amount due to external factors, such as the Great Depression in 1929.
One of the major factors that led to the failure of collective security was the ambiguity and the overall lack of military forces of the League. The League of Nations could theoretically call for military action if necessary, but it did not have its own armed forces; furthermore, none of the member states wanted their sovereign forces under international control. The League was intended to practice collective security by imposing moral pressure and economic sanctions to solve disputes. Without any true military power, however, the League encountered problems that prevented them from confronting machinery, which especially arose during the Manchurian Crisis in 1931. The League was unable to impose economic sanctions to solve the Manchurian Crisis; but more importantly, it was unable to use military solutions. Britain had the naval resources to confront the crisis, but due to the overall lack of organization for machinery, Britain was unwilling to act. Thus, the aggressor was able to get away with the aggression, and China, who had originally appealed to the League, never received military or economic support. Military power - or one lack thereof - was a huge importance to the failure of collective security to maintain peace between 1920 and 1935.
Major powers such as USA, USSR, and Germany were absent from the first formation of the League of Nations, which lowered the League's political, social, and economic powers. USA was the most powerful economic country in the world, and it had withdrawn into isolationism. If USA had been part of the League, it would have provided real weight to any economic sanctions imposed by the League. USSR was excluded, which weakened the League because it appeared to only contain capitalist countries. The absence of Germany revealed that the League ignored the fact that Germany remained a strong power at the end of WWI; Germany was militarily defeated in the west but not in the east, which meant that its expansionist politics or economic power had not been evaporated. Had these powers been included in the League, there may have been a possibility of more success at keeping peace via collective security during this era.
The failure of collective security was demonstrated particularly in the Ruhr Crisis of 1923, the Manchurian Crisis of 1931, and the Abyssinian Crisis of 1935, where the power of the League was undermined and rendered powerless in all three instances. During the Ruhr Crisis, France attempted to seize Germany's reparation payments using force; the French and Belgians moved troops into the Ruhr Valley to take what they were owed. This compromised the League's credibility because it appeared that member states were allowed to take independent action whenever it suited them. During the Manchurian Crisis in 1931, China appealed to the League but received no support, therefore leaving the aggressor unpunished for its actions. Collective security was practically finished during the Abyssinian Crisis in 1935, when Mussolini invaded Abyssinia. A permanent member of the League had become victorious through violence and war, and collective security failed to withhold this crisis. Therefore, collective security finally failed and ended after the Abyssinian Crisis - a crisis that was one of many in which the League of Nations was gruesomely undermined and humiliated.
When one views in hindsight the potential ability of the League to uphold collective security to maintain peace, one can see that there were some factors that immensely prevented the success of collective security. One of these factors was the Great Depression that began in 1929. This economic disaster influenced many countries' decisions, making them act by national self-interest rather than in the interest of collective security. The Great Depression was also responsible for preventing economic sanctions as a possible force to subdue countries that opposed the League. So, although historians may argue that the League was doomed to fail to uphold collective security and maintain peace during 1920 through 1935, there were factors that severely limited any chances of success.
Due to poor military power, the absence of major powers, and multiple instances of undermining, the League of Nations was unable to keep the peace between 1920 and 1935. However, it is difficult to say how successful collective security would have been had the Great Depression not coincided with this era.
