Clinton's address at Washington's Brookings Institution was billed as an opportunity to outline the Obama administration's agenda for the new U.N. General Assembly that begins next week.
But she used the appearance to respond to criticism of President Obama's decision announced Thursday to scrap plans to put a U.S. long-range missile defense system in Poland and the Czech Republic, in favor of a layered regional system aimed at countering the medium range missiles Iran currently has.
The abrupt end to the Bush administration project is said to have unsettled U.S. European allies and has triggered harsh criticism from U.S. conservatives that the Obama administration is wilting in the face of Russian objections to the program.
Clinton said the new plan, relying heavily at first on sea-based missile interceptors, will mean deployment of an effective defense system years earlier than what she said was an unproven and costly approach by the previous administration.
She reiterated the U.S. NATO commitment to the Warsaw and Prague governments and insisted Russian opposition hadn't played into President Obama's calculations.
"This decision was not about Russia. It was about Iran and the threat that its ballistic missile program posed," Clinton said. "And because of this position, we believe we will be in a far stronger position to deal with that threat, and to do so with technology that works and a higher degree of confidence that what we pledged to do, we can actually deliver."
Clinton will meet on the sidelines of the U.N. General Assembly next week with counterparts from the other permanent Security Council member countries and Germany in advance of a potentially-decisive meeting October first between diplomats of the P-Five-Plus-One and Iranian nuclear negotiator Saeed Jalili.
She suggested again that time is running out for Iran to put to rest concerns that its nuclear program is weapons-related, or face further international sanctions and isolation. She suggested the Tehran government's crackdown on internal dissent only enhances doubts about is intentions.
"Since June, we have seen the Iranian government engaged in a campaign of politically-motivated arrests, show trials and suppression of free speech," Clinton said. "The Iranian government seeks a sense of justice in the world, but stands in the way of the justice it seeks. None-the-less we remain ready to engage with Iran - not as an end in itself but as a means of addressing the growing concerns that we and our international partners have about Iran's actions, especially on the nuclear issue."
The secretary said non-proliferation and disarmament, with the eventual aim of eliminating nuclear weapons altogether, are key U.S. priorities at the United Nations, and will be underscored by unprecedented plans for President Obama to chair a special Security Council meeting on the subject next week.
She also cited food security and the empowerment of, and prevention of violence against, women as key American aims within the world body. While the United States took a skeptical approach toward the United Nations in recent years, Clinton said in the 21st century "we ignore and walk away from it at our own peril."