President Barack Obama, who is on a visit to Argentina, has promised to release secret files concerning the US role in the military coup there 40 years ago that installed one of the region's most brutal regimes.
He was speaking after talks with Argentine President Mauricio Macri.
Mr Obama said US military and intelligence files from the era would be declassified for the first time.
He arrived in Argentina after a historic three-day visit to Cuba.
President Macri said the visit marked the beginning of "new mature and intelligent relations" between the two countries.
US-Argentina relations were less close during the previous left-wing governments of Mr Macri's predecessors, Cristina Fernandez de Kirchner and Nestor Kirchner.
Mr Obama praised the reforms implemented by Mr Macri's centre-right government since he came to power in December.
"President Macri is a man in a hurry. I'm impressed because he has moved rapidly on so many reforms to reconnect Argentina with the world community," he said.
Mr Obama said that by offering a more stable financial scenario to foreign investors, Mr Macri would attract billions of dollars of investment from US businesses.
"Argentina needs to be a critical partner with us. It is one of the biggest, most important countries in the region."
Earlier this month, Argentina signed an agreement with US hedge funds to settle a 15-year dispute over its failure to repay billions of dollars worth of bonds.
Ms Fernandez had refused to negotiate with the investors, labelling them "vulture funds".
'Our darkest chapter'
Mr Obama expressed hope that his gesture to release secret files of the 1970s would help mend relations between the two countries.
"We are absolutely determined to do our part as Argentina continues to heal and move forward as one nation, and I hope this gesture also helps to rebuild trust that may have been lost between our two countries," he said.
His presence in the country coincides with the 40th anniversary on Thursday of the military coup, which Mr Macri describe as "the darkest chapter in our history".
Some groups are planning protests because of the support American governments gave to the military coup of 24 March 1976.
Some 30,000 people are estimated to have been killed during the military period, which lasted until 1983.
Thousands of other people were illegally detained and tortured in what became known as the "Dirty War".
General Jorge Rafael Videla, who led the coup, died in jail in 2013, serving a sentence for human rights violations.
Mr Obama said US foreign policy had learned from the mistakes of the past, including its involvement in the Argentina coup.
"I will visit a memorial to the victims of the Argentine dictatorship," he said.
Brazil crisis concerns
President Macri and President Obama also announced a partnership to tackle drug trafficking and money laundering.
And both leaders expressed concerns about the political crisis in Brazil, Argentina's biggest neighbour and commercial partner.
"We hope Brazil solves its political crisis. Their democracy is sufficiently mature.
"We need a strong and effective Brazil for our own economies and world peace," said Mr Obama.
Mr Macri said that the Brazilian crisis had a potential impact on the Argentine economy.
But he expressed his confidence that "Brazil will come out of this crisis strengthened".