There have been two major changes in the permanent members of the United Nations Security Council since 1945, which two countries were involved?

When the United Nations was formed in 1945, there were five countries who occupied the permanent membership seats, U.S.S.R, Republic of China, United States, United Kingdom and Republic of France. These nations were the major allied victors of the Second World War. The two nations involved in the changes were the Republic of China and U.S.S.R.

The first major change occurred in 1971, when the United Nations issued a resolution recognizing the People's Republic of China as the legitimate nation to hold one of the seats on the Security Council. In 1949, the Chinese Civil War culminated in the expulsion of the Nationalist Republic of China's forces from the mainland and retreat to Taiwan. The Republic of China has maintained control over the island since then, with the People's Republic of China viewing Taiwan as a renegade province. A U.N. resolution in 1971 modified the rules to recognize the government on the mainland as the rightful representative.

When the U.S.S.R. was formally dissolved in 1991, the United Nations recognized the Russian Federation as the lawful successor to the permanent seat on the Security Council.

The permanent members are all nuclear powers, with some of the largest military expenditures in the world. Permanent members possess veto power in the council on any substantive matters (resolutions). A veto will stop a resolution even if there are an ample number of yes votes for the resolution to pass.