Afghan and Tajik leaders have inaugurated a new bridge linking their countries across River Panj.
Critics fear the bridge may help the spread of extremism - and drugs
The $37m (£18.4m) project, a US gift, replaces an intermittent ferry service across the fast-flowing river.
"This bridge of friendship is an historic event," said Tajik President Emomali Rakhmon at the ceremony.
His Afghan counterpart, Hamid Karzai, said he hoped it would improve economic and trade co-operation "between the countries and the region".
The bridge - nearly 700m (yards) long and 11m across - will replace an unreliable ferry which was the only way of getting across the River Panj.
US Secretary of Commerce Carlos Gutierrez joined the Afghan and Tajik presidents at the opening ceremony.
Laughing and tapping each other on the shoulders, the two leaders cut the symbolic ribbon and opened the brand new bridge linking their countries.
President Emomali Rakhmon said the bridge would become "an important link in the development of trade between the two countries".
"The opening of this bridge provides a chance for boosting trade not only with Central Asian countries, but also with China, Russia and the Gulf countries," he said.
The bridge, according to the Americans, is the missing link of the ancient silk road.
It could turn Afghanistan into a major transit point for goods and link the whole of central Asia to Pakistan's port of Karachi.
But the problem is that the ancient silk road is today a major drug trafficking route, reports the BBC's Natalia Antelava who attended the ceremony.
Many here fear that drugs and instability will be Afghanistan's most likely export, she says.
The heroin that makes its way to Europe comes mostly through Tajikistan.
The opening of the bridge, critics say, could increase the drug trafficking further.
Acknowledging this, the Tajik leader said: "Everything must be done to ensure that it is not used by drug traffickers or people smugglers."
And the US secretary of commerce dismissed the fears.
"America," Mr Gutierrez told the BBC, "cannot stop developing trade just because it's concerned about the drugs."
But the money is nothing compared to the political investment made by the US, says our correspondent.
For Washington this is a showcase project, a proof that things can go right for Afghanistan and its neighbours, she says.
The bridge does offer a link to the outside world that Afghanistan has long needed.
But with the security situation deteriorating and with drug trafficking on the rise, the question is whether bringing Afghanistan closer to the world will help to solve its problems, or will it, in fact, allow them to spread, our correspondent says.