The road to rock bottom for relations between Brazil and the United States, a dispute that now threatens business ties between the two Western Hemisphere economic giants, began in Brasilia in March.

U.S. Secretary of State Hillary Clinton was trying to convince Brazilian President Luiz Inacio Lula da Silva to drop or postpone his controversial efforts to negotiate a nuclear deal with Iran, and support a new round of sanctions instead.

Lula refused. Then, he dropped a diplomatic bombshell, telling Clinton he was worried Iran would become another Iraq -- that is, that the United States was on a path to war, according to sources familiar with the exchange.

Clinton bristled, replying that Lula wasn't taking into account the fact that Barack Obama was now president, rather than George W. Bush, the sources said on condition of anonymity because of the sensitivity of the talks.

The meeting ended badly. Two months later, Lula was standing in Tehran alongside Iranian President Mahmoud Ahmadinejad, trumpeting a fuel swap deal for Iran's nuclear program -- an agreement that was immediately ignored by the United States and most other major world powers, who pursued sanctions anyway.

Today, the fallout from Iran remains worse than either side will acknowledge publicly.

Brazilian officials complain in private that their good-faith efforts to broker a peace deal were brushed aside, with little regard for Brazil's status as an emerging power. Meanwhile, officials in Washington are angry that an ostensible regional ally interfered on an issue they feel is one of the biggest long-term threats to global security.

The bottom line, and the issue of most concern to investors, is that there is now a real risk of a longer-term drift between Brazil and the United States that could have a negative effect on bilateral trade and business.

"I worry about it," Representative Eliot Engel, the Democratic chairman of the U.S. House Foreign Affairs subcommittee that covers Latin America, said in an interview. "I think it's Lula's policy to try and push the countries apart. He wants the U.S. to be less influential."

Nerves between the two countries were already raw following other disputes including a narrowly averted trade war this spring over U.S. cotton subsidies; differences over the handling of last year's coup in Honduras; and the disappointing outcome of the climate summit in Copenhagen in December.

The clashes have been especially disappointing since both sides had anticipated improved ties under Obama, who made a point of fawning over Lula last year, calling him "my man" and "the most popular politician on earth."

These days, an upper-level source in Brasilia described the chill in bilateral ties this way: "They're in the freezer."

Trade Ties Suffer

The countries' diplomats still have regular, polite contacts, and investment flows in both directions at ever-greater numbers. Yet there are signs that the bad blood is starting to damage business ties:

• The U.S. Congress is now less likely to reduce tariffs on imports of ethanol, of which Brazil is the world's No. 2 producer behind the United States, Republican and Democratic sources on Capitol Hill told Reuters.

Lobbyists for the U.S. corn ethanol industry including retired Gen. Wesley Clark have seized on the Iran issue as a reason to leave tariffs in place. Clark told an industry conference in June that it was a mistake to buy energy from any country -- specifically, Brazil -- that doesn't "see eye to eye" with the United States on national security issues.

"This would have been a tough issue under any circumstances," one source said. "After Iran, it's become almost impossible" to lift the tariff.

• Negotiations have stalled for a Trade and Investment Framework Agreement, or TIFA, between the U.S. and Brazil. A TIFA basically provides a road map for more in-depth free trade negotiations between countries, and the issue has been a priority for diplomats for both countries for some time.

A source who