Explain why the League of Nations was hailed as a success during the 1920s but was considered a failure by 1936.

The league had two main aims. First, collective security, the members of the league will act together if one state attacked another. Second, to encourage international cooperation in the economic and social sector. In 1920s League was able to somewhat achieve both its political and socio-economic aims.
Major achievements in socio-economic sector include:

• Success of International Labor Organization in improving the conditions of workers by persuading governments to introduce minimum working hours and wages.
• Half a million prisoners of war returned to their homes with The Refugee Organization.
• The Mandate Commission effectively supervised the governments of territories taken from Germany and Turkey.

Several political issues were referred to League in the 1920s, some of these are:

• Dispute over the Aaland Island between Finland and Sweden in which verdict went in favor of Finland (1920).
• Upper Silesia was partitioned between Germany and Poland after rival claims in 1921.
• When Greeks invaded Bulgaria they were forced to withdraw and asked to pay for damages.
• In South America disputed were resolved between Peru and Columbia, and between Bolivia and Paraguay.

However, not all verdicts were respected first in the case of rival claims for Vilna between Poland and Lithuania in 1920 and second, during Corfu incident of 1923. Which was not the good sign for League. Also till now none of the conflicts mentioned threatened world peace, and none of the verdicts went against major powers. But by the mid-1930s things started to take the ugly turn. Many challenges occurred during this period and in almost all cases League of Nation failed to deliver.

• The Great Depression: the situation was under control up until the great depression. This economic crisis of 1929 led to mass unemployment and falling living standards. Taking advantage of this right wing governments came to power in Germany, Japan, and Italy. Actions taken by them in international politics (mentioned below) severely hampered the prestige of League of Nation.
• The invasion of Manchuria by Japan in 1931: No economic sanctioned were imposed on Japan let alone the military options.
• The failure of World Disarmament Conference of 1932-33: Failure of this conference gave Hitler an excuse to leave the League.
• The invasion of Abyssinia by Italy in 1935: Only halfhearted sanctions were imposed that too were abandoned after few weeks.

After 1935 League of Nation was not taken seriously and the main reason for its failure was that when aggressive states like Japan, Italy, and Germany defied it, it took no appropriate action. This led to the loss of confidence among smaller states.