Since the United States invaded Iraq in 2003 without U.N. support, the country's engagement with the world body has wavered between grudging participation and downright hostility. When he assumed office, President Obama signaled an immediate change in tone by appointing Susan Rice, a national-security expert who worked on terrorism and Africa during the Clinton administration, as the American U.N. ambassador. She has now held the post for nearly six months; Rice spoke to NEWSWEEK's Andrew Bast recently about Washington's new position and what it means for the world's most threatening challenges. Excerpts:

Bast: How have you changed the U.S. approach at the U.N. since taking over as ambassador?
Rice: In implementing President Obama's commitment to engage with the rest of the world, we're trying to demonstrate—in tone and in substance— new form of American leadership, one that seeks partnership and cooperative solutions rather than confrontation for confrontation's sake. One that recognizes that our national interests can often be best served by bringing other countries together to achieve the goals we seek.

Is Resolution 1874, passed June 12, strong enough to contain North Korea?
It is the strongest package of sanctions in effect against any country in the world today. The aim is to constrain North Korea's ability to pursue its missile and nuclear programs by taking away the revenue that it has derived in the past from arms exports, [by] inspecting vessels believed to be containing contraband cargo, and [by] freezing the assets of more North Korean companies and individuals. So this is a strong regime that we believe, when fully implemented, will bite in North Korea.

The first ship that was targeted after the resolution has in fact made its way back to North Korea, correct?
There was a ship of possible concern. We and others followed its progress, and it has indeed returned to North Korea.

Will the Security Council be able to keep Iran from developing nuclear weapons?
The ball is in Iran's court. The reality is that in April, the five permanent members plus Germany delivered a clear-cut proposal that offered Iran two paths: either end its illicit nuclear program and rejoin a responsible community of nations, or continue to pursue its nuclear-weapons program and find itself increasingly isolated. That offer remains in place, but as President Obama and the other members of the G8 reiterated in Italy, it is not an indefinite or open-ended offer.

Secretary-General Ban Ki-moon has talked about a "new horizon" of peacekeeping. What is the future for the blue helmets?
There are more U.N. peacekeepers deployed today than ever before around the world—about 115,000 military, police, and civilians in some 16 operations. U.N. peacekeeping costs us about 12 cents on the dollar compared to what it would cost us to act if we did so by ourselves. It's a valuable tool, and it's one that serves our interests. But there are some real challenges at present. It is overstretched with this large number of deployments and the complexity of many of these operations, particularly in places like Sudan and Congo.

As China emerges as more of an economic and military power, how do you see its role evolving at the U.N.?
China is clearly playing a more active and multifaceted role at the U.N. than it was, say, a decade or two ago. We take China's role seriously. We don't always agree. And where we differ, we try to see how we can narrow those differences. Where we are able to find common ground, as we've done of late with respect to North Korea, we can make important progress.

Has the U.S. paid all its back dues to the U.N.?
Congress just passed and the president signed legislation that included very substantial payments. So, for the first time in a while, we are in good shape and in a position to be meeting our obligations in full.

President Obama has called the U.N. "indispensable and imperfect." What makes the U.N. imperfect?
It's a very accurate characterization. The U.N. is playing a crucial role in peace and security in various different parts of the world, but it is imperfect on multiple levels. There continue to be instances of less-than-high-quality management, waste of resources, and in some egregious instances, such as the Oil-for-Food Program, outright fraud and corruption. Real progress has been made in recent years, but this will continue to be a challenge and an important priority for the U.S. to help to reform the institution so that it is worthy of the resources we commit.