How does this event contribute to the issue of nuclear proliferation?

The correct answer is WWII

In August 1945, the world experienced the devastating effects of a nuclear bomb. On that occasion, the United States of America (USA), to demonstrate its warlike power, dropped nuclear bombs in the Japanese cities of Hiroshima and Nagasaki. The consequences were dire: environmental disasters, hundreds of thousands of people died, the development of burns, blindness, deafness and cancers.

Faced with the destructive effects of nuclear weapons, the countries that won the Second World War, using the ideological discourse of inhibiting the expansion of these weapons, were the main responsible for the elaboration of the Nuclear Non-Proliferation Treaty (NPT). This treaty was signed in 1968 and came into force in 1970, and currently has 189 countries.

According to NPT regulations, only nations that blew up the atomic bomb before 1967 are entitled to possess such weapons. These countries are: United States of America, Russian Federation (which succeeds the Union of Soviet Socialist Republics), United Kingdom, France and China. Ironically, these are the five countries with voting power on the Security Council of the United Nations.