Dear student,
United Nations was formed as an international peacekeeping organization. It was a forum for resolving conflicts between nations. Before the UN, there was an organisation named League of Nations, which failed to stop the second world war. So, the League of Nations was replaced by the UN to stop any further wars and resolve issues between the countries.
But if UN had not been established, there would the possibility of World War three, mass killings of man-force, no effective peace control measures, etc.
The UN had 51 countries when it was formed but now it has a total of 193 countries.
Regards,