After World War One what did the United Nations do that resulted in conflict in the middle east

Let me tweak your question just a little bit. After World War I it was the League of Nations that was created, and actions by the League of Nations created issues in the Middle East. [Note: The League of Nations didn't endure. After World War II, there was a second attempt to form an international body, and that's the one we still have today, called the United Nations.]

As a result of internal problems it had been having and its losses in World War I, the Ottoman Empire collapsed. Headquartered in Turkey, the Ottomans had controlled most of the Middle East before that. Following the war, the winning European allies from World War I divvied up territory in the Middle East as areas over which they would serve as temporary governors to get things going again. These "mandates" were granted by the League of Nations, and Britain and France each got some "mandate" territory.

There's a lot more detail we could go into, but during the Mandate years many of the situations of conflict that we see in the Middle East today had their roots.