Iran and a group of six nations led by the United States have agreed to a historic accord. (Reuters Photo)

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Iran and a group of six nations led by the United States said they had reached a historic accord Tuesday to limit Tehran's nuclear ability for more than a decade in return for lifting international oil and financial sanctions.The deal culminates 20 months of negotiations on an agreement that President Barack Obama had long sought as the biggest diplomatic achievement of his presidency. Whether it portends a new relationship between the United States and Iran - after decades of coups, hostage-taking, terrorism and sanctions remains a bigger question.Obama, in an appearance at the White House that was broadcast live in Iran, began what promised to be an arduous effort to sell the deal to Congress and the U.S. public, saying the agreement was "not built on trust - it is built on verification."But Obama made it clear that he would fight to preserve the deal in its entirety, saying, "I will veto any legislation that prevents the successful implementation of this deal."Not everyone was celebrating the signing of the accord. Israel's prime minister, Benjamin Netanyahu, called it a "mistake of historic proportions" that would create a "terrorist nuclear superpower."The United States secured restrictions on the amount of nuclear fuel that Iran can keep in its stockpile for the next 15 years. It will require Iran to reduce its current stockpile of low enriched uranium by 98 percent, most likely by shipping much of it to Russia.That measure, combined with a two-thirds reduction in the number of centrifuges spinning at Iran's primary enrichment center at Natanz, would extend to a year the amount of time it would take Iran to make enough material for a bomb should it abandon the accord and race for a weapon - what officials call "breakout time."Iran's president, Hassan Rouhani, made a brief statement, saying that the Iranian people's "prayers have come true." A senior Iranian official in Vienna, speaking to reporters on the condition of anonymity, called the agreement "a good deal that the Iranian people will support," but added that he was uncertain how it would "translate in the economics of the country."