How has the United Nations evolved with respect to its purpose and principles?

The original goal of the United Nations (UN) was to maintain global peace and security and to develop friendly nations. Developers of the UN expected all nations involved to work closely together towards humanitarian goals and preserving peace. However, the Cold War immediately created friction between the Soviet Union and the other nations that made it difficult for the UN to fulfill its original goals. Since then, the UN has fluctuated with respect to assertiveness in restoring peace. One of the original goals had been to establish a UN military, created by the Military Staff Committee and directed by the Security Council. Yet, the committee was unable agree about any plans for the military, mostly due to the Soviet Union's opposition. Similarly, no agreements were reached concerning disarmament, particularly nuclear disarmament. A UN military was not established until the Korean War, with the US providing most of the fighting power.Problems have arisen when members of the Security Council were able to veto the council's actions. To remedy the problem, the "Uniting for Peace" resolution was adopted in 1950, which gave the General Assembly permission to act when the Security Council was paralyzed by veto. However, the resolution doesn't provide a basis for using force to restore peace; therefore, the resolution has "never developed workable enforcement machinery" ("United Nations," The Columbia Encyclopedia, 6th ed., 2016). Since the resolution, the UN was most assertive in "enforcing the Gulf War cease-fire resolutions in the early 1990s" but has since then not been as assertive in establishing or maintaining peace in "Somalia, Bosnia, Haiti, Sierra Leone, and Côte d'Ivore" ("United Nations").
https://digitalcommons.law.yale.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=3278&context=fss_papers

https://www.un.org/en/sections/history/history-united-nations/