Trump presses UN for reforms, but they won't be easy

President Donald Trump told the United Nations General Assembly on Tuesday that he will "always put America first" and warned that the "U.S. can no longer be taken advantage of and get nothing in return."

Trump's debut speech included a complaint that the U.S. is paying more than its fair share of the cost of maintaining the 70-year-old body, and he urged U.N. officials to figure out how to do more with less.

"The United States is one of 193 countries in the United Nations, and yet we pay 22 percent of the entire budget and more," Trump said. "The United States bears an unfair cost burden, but to be fair, if it could actually accomplish all of its stated goals, especially the goal of peace, this investment would easily be well worth it."

Many U.N. officials, including Secretary-General Antonio Guterres, agree that changes are long overdue. But compared with the rest of Trump's agenda, reforming the U.N. will make rewriting the U.S. tax code and overhauling Obamacare look easy.

To begin with, the U.N is not a single institution so much as it is a sprawling global network of dozens of humanitarian and development agencies. By design, the decision-making process is distributed among a wide range of countries and constituencies that may have little in common and opposing views and interests.

Trump has complained repeatedly that the United States is paying more than its fair share of the U.N's budget.

As the largest single contributor to the U.N. budget, the U.S. pays about 25 percent of the organization's regular operating budget and more than 28 percent of a separate peacekeeping budget. The U.S. has yet to make its payment this year, prompting fears that it may cut its annual contribution.

But measured as a share of its economy, the U.S. portion is about average for large developed countries. Its roughly $10 billion in contributions last year amounted to about six one-hundredths of 1 percent of the more than $18 trillion in 2016 U.S. gross domestic product.

The president also praised Guterres, who has said he shares Trump's vision for a less wasteful U.N. to "live up to its full potential." The U.S. has asked member nations to sign a declaration on U.N. reforms, and more than 120 have done so.

But enacting those reforms will be as challenging as the multiple missions the global body has taken on, everything from peacekeeping in some of the world's most difficult conflicts to eliminating hunger and poverty around the world.

WATCH: Trump tells U.N. the U.S. bears an 'unfair burden'