UNITED NATIONS — Nearly a year before the world woke up to images of Syrians dying in a large-scale chemical weapons attack, Samantha Power was quietly pushing President Obama for a military strike to stop what she calls the “grotesque tactics” of President Bashar al-Assad. For a fleeting moment this month, it seemed she had prevailed.

Now Ms. Power, a former senior aide on the National Security Council and a former war reporter who emigrated from Ireland, must negotiate for peace in a new public role as Mr. Obama’s ambassador to the United Nations. The president’s abrupt decision not to use force in Syria has thrust her into the middle of contentious talks to create a United Nations Security Council resolution mandating the elimination of Mr. Assad’s chemical arsenal by the middle of next year.

She will be on the spot on Monday, her diplomatic debut, as Mr. Obama arrives in New York for the United Nations General Assembly. A woman known for her closeness to the president and the soaring prose of her Pulitzer Prize-winning book on genocide, “A Problem From Hell,” Ms. Power is the lead American negotiator in the difficult, gritty business of arguing with the Russians, Syria’s patrons, who have already rejected the notion of using force if Mr. Assad does not comply.

Even her supporters wonder if the untested Ms. Power will be tough enough, a question with big implications. Secretary of State John Kerry will work with her on the resolution, but her role is so central that her performance — in her first weeks on the job — will help determine America’s future course in Syria.