Israel was established in 1948 and the UN was established in 1946. Unless Israel possesses a very special DeLorean, Israel could not have founded the United Nations. Conversely, in 1947, the UN General Assembly passed UNGA Resolution 181 (II) which gave the legal legitimacy to the Zionist movement in Mandatory Palestine to declare independence as Israel. The Zionists followed through on this six months later by declaring the independence of the State of Israel.
The United Nations did not "establish" or "create" Israel. Zionist Jews created and established Israel.
The phrasing of the question misconstrues the actual history concerning the authority of the United Nations General Assembly Resolution 181. Most people incorrectly believe that UNGA Resolution 181 (II), passed on November 29, 1947, "created" a Jewish State (Israel) and an Arab State (Palestine) on the map. This is not the case. What the UN Resolution does provide for is the permission for the different ethno-religious groups in Mandatory Palestine to declare a state. In the case of most mandates, the understanding was that the mandate would eventually become independent as one new state, like Iraq or Syria had. The case of Palestine was therefore unique and needed the permission to deviate from the traditional path of independence. Because of there being two states, provisional borders had to be provided, which is why a map was used, but those borders would only come into play if both sides decided to remain at peace.
The Jews acted on the permission granted to them by UN Resolution 181 by declaring independence on May 14, 1948 (nearly six months after the Resolution was passed) and therefore creating the State of Israel.
The UN didn't establish the State of Israel. Late in 1947, the UN passed a resolution to
partition (divide) the British Mandate in Palestine into two parts, one to be administered
by the Arabs, the other to be administered by Jews, and to let the British go home.
The British and the Jewish authorities accepted the plan, the Arab authorities didn't. On
the day the partition resolution took effect, in May 1948, the Jewish authorities declared
their section to be a sovereign state, and the Armies of several of their Arab "neighbors"
invaded the Jewish section, to prevent it from becoming anything. Israel had no military
force at the time. But that was 68 years ago, and Israel is still there.
Israel became a member of the United Nations on March 4, 1949 according to UNSC Resolution 69.
Israel has been a member of the United Nations since the UN General Assembly voted on the May 11, 1949 with UNGA Resolution 273 (III) to admit Israel to UN membership as a "peace-loving country".
United Nations Day isn't solely an American holiday but a holiday celebrating all of the peoples that came together to form the United Nations. It was the United Nations General Assembly that established the holiday in 1948. The date set for the holiday is October 24th, the same day that the United Nations Charter was adopted in 1945. United Nations Day is just one of the events that take place during United Nations Week which runs every year from October 20-24th. United Nations Day isn't just a holiday to celebrate the beginning of the organization. Its purpose is more to uphold what the United Nations stands for; international economical development, security, social progress, human rights and world peace. Though none of the dozens of UN member states are required to recognize the holiday, the General Assembly did request it in 1971. Since 1946, even before its official inception, every year the U.S. President has proclaimed a United Nations Day. There are also various learning institutions and international schools across the world that recognize and celebrate United Nations Day as a day to show pride in their cultural diversity.
International Year of Deserts and Desertification
1945
2002
1955
1946
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1965.
The Dominican Republic is a member country of the United Nations. The United Nations was established in October of 1945, and the Dominican Republic was one of the original members that joined that year.
International Year of Sanitation
Original Answer: Because they are decades away. Improved: The United Nations are not a country. They couldn't have "joined" the League of Nations. The League of Nations was the predecessor of the United Nations - the League of Nations failed, and so, many years later in 1942 (I believe it was that year) they founded the United Nations as an improved "League of Nations."