After Italy declared war on Great Britain and France on June 10, 1940 Turkey remained neutral, despite the fact that the treaty with Western powers obliged it to enter the war in such circumstances. Turkish government referred to the Second Protocol attached to the Treaty of Mutual Assistance which made possible for the Turkish side to ignore their obligations in case a threat of armed conflict with Soviet Union. However it is still not known if this was real reason for Turkish decision. The aim of this article is to review interpretations of Turkish attitude that have been present in historiography since the war. It includes short-term and long-term factors of Turkish decision from June 1940. In addition, attention was concentrated on British intelligence sources, which, in relation to the period between spring and summer of 1940, have not yet been taken into account by scholars when trying to determinate Turkish motives.