The primary mission of the United Nations is to help different countries, such as the United States, unite and maintain world peace. Also at the United Nations, country leaders address world issues such as climate change.
The framers of the Constitution devised a system wherein no one branch of the Government would be able to dominate.
This means that, by design, each branch of the US Federal system is co-equal.
This ensures that before action is done on behalf of the people of the US by the Federal government, there are adequate safeguards to ensure that what is being done is done within the bounds of the Constitution.