Nikki Haley was a stellar ambassador to the United Nations, and replacing her will not be easy. There are more pitfalls at the U.N. than there are in other diplomatic posts, and they can defeat ambassadors of unexceptional talent.

President Trump needs to find a replacement willing to confront the many, and often multilateral, perfidies of deployed against the United States at the U.N., and succeed as Haley did at prosecuting an enlightened policy of "America First."

[Read: The top 10 contenders to replace Nikki Haley at the UN]

Trump's choice must serve American interests without alienating the rest of the world. For example, it would not be best to replace her with an ambassador in the mold of John Bolton. He has imposed much-needed discipline and direction in Trump's national security efforts at the White House, but his tenure as U.N. ambassador produced much acrimony. Haley was just as tough and unflinching, but she managed to be so with an deft elegance that gained respect without prompting enmity or disdain.

Bolton and Haley both put America first, but Haley saw that American interests are best served by partnerships. Building strong personal relationships with allied, neutral, and even adversarial ambassadors, she made the U.S. a dealmaker and broker at the international table.

On North Korea, Haley was able to fortify international will around perhaps the most stringent sanctions regime ever enforced. When publicizing President Bashar Assad's atrocities in Syria, the world paid notice, and Russia squirmed as Haley established the moral case for American military reprisals. On Israel, Haley made sure the terrorists of Hamas were made accountable. Haley achieved these successes because she had earned respect from her foreign counterparts.

A U.S. ambassador must represent American interests as determined by the president. He or she needs the strength to avoid diplomatic capture, by which weaker souls end up representing the international community rather than their master at 1600 Pennsylvania Ave. At the same time, he or she needs deftness and an evident willingness to listen. Too many of our recent U.N. ambassadors have taken up residence at U.N. headquarters, only to be seduced by the parties and prestige that come with their new position. Too often, U.S. ambassadors have closed their eyes to the U.N.'s grave failings. That is intolerable.

American treasure has been poured too often into U.N. organizations that work against America, or for other vile purposes, or both. U.N. ambassadors need to be willing to remind their counterparts that they are servants not of a global body, but of their own people. That distinction defines democratic values. More than that, it defines the better possibility of the U.N.