Question

He within the United Nations exists to prevent military conflicts between nations. The widespread devastation that World War II caused prompted world leaders to create this UN division, whose main purpose is to prevent war. The United States is member of the Security Council.

Answer 1

You seem to be answering your own question within the sentenced you provided.  You are describing the United Nations Security Council.

Let me give you a little more detail about the UN Security Council:

The Security Council of the United Nations is responsible for the maintenance of international peace and security.  It is the only group or agency within the UN that can make decisions which are binding on all members of the United Nations.

The Security Council has the authority to send peacekeeping forces into conflict zones and to impose sanctions on member states of the UN that are deemed to be acting against the cause of peace.

The Security Council also approves new member states of the UN and plays a role in the election of judges to the International Court of Justice.