The League failed as a result of a number of different factors.
A key factor in any explanation of the League’s failure is British and French self-interest: they looked after their own interests rather than the League’s. For example, when Japan invaded Manchuria both countries were more concerned to preserve their empires in South-East Asia than help China. Whilst France refused to disarm as it feared Germany so much. Then, when Italy invaded Abyssinia, the British and French foreign ministers (Hoare and Laval) proposed a secret deal with Mussolini ignoring their responsibilities as permanent members of the League’s Council. Though with regard to Germany, Britain was also increasingly concerned that the Treaty of Versailles was unfair. This was seen, for example, in Britain’s attitude at the Disarmament Conference.
Another critical factor was the absence of major countries such as America, and at least to begin with, Germany and Russia. Russia was the only major power in a position to send troops against the Japanese, whilst America was Japan’s biggest trading partner and so, sanctions would have hurt Japan if America had been a member of the League. And during the Abyssinian crisis, the League debated whether to apply oil sanctions on Italy as it feared America would not support them. American leadership would also have been important in the Disarmament Conference too. After all, the Washington Conference in 1921 led to agreements to limit the size of the American, Japanese, British and French navies.
Sanctions as a way to apply pressure on an aggressive country, either weren’t used or didn’t work, and this is another important factor. Sanctions were not used at all in the case of Manchuria, League members couldn’t even agree to banning arms sales. Sanctions were not even discussed at the Disarmament Conference when they could have been used to force countries to disarm. Whilst in the Abyssinian crisis, though arms sales to Italy were banned, as well as the sale of rubber, tin and metals, the crucial issue to ban the sale of oil (without which the Italian war effort would have quickly ground to a halt), was not made.
The League might have been more effective if it had had its own army. In Manchuria it would have been costly, and taken a lot of time, to send troops from European countries. But this was not the case with Abyssinia where British and French forces could have been used.
The speed in which the League made decisions, as we have seen to some extent, was also a factor. In Manchuria it took a full year before the report into the Japanese invasion of Manchuria, which said the Japanese had acted unlawfully and that Manchuria should be returned to China, was presented to the League. The Disarmament Conference didn’t meet until the beginning of 1932, more than a decade since the League had been set up. Even in the Abyssinian crisis, there was unnecessary dithering, unnecessary delays.
The backdrop to the 1930s was, of course, the Great Depression and this was another critically important factor. As well as making League members less willing to impose sanctions for fear of harming their own trade, it also led to a rise in dictators. The Great Depression was a key factor in the Japanese invasion of Manchuria as they sort new markets to boost their economy, and it helps to explain the reluctance on the part of European governments to act. Similarly, Mussolini’s invasion of Abyssinia is partly explained by his desire to gain fertile lands and mineral wealth, whilst the reluctance to impose sanctions is partly explained by the impact it would have on British and French jobs. Even disarmament was affected by the Great Depression as jobs would have been lost at a time when unemployment was rising rapidly. However, the most important factor affected by the Great Depression, was the rise to power in Germany of Hitler. Votes for the Nazi Party rose in line with the rise in the numbers of Germans unemployed. And Hitler was to become the key factor in any explanation of WW2.
So, we can see that a number of different factors play a part in explaining why the League failed in the 1930s. It is important that you understand the significance of each of them, but it is also important that you see how they link to each other.