A US federal judge has ordered Iran to pay $2.65bn to families of 241 soldiers killed in the 1983 bombing of a military barracks in Beirut.
A 2003 ruling by another court in found that Iran provided financial and logistical help for the bomb which it said was carried out by the Hezbollah group.
Hezbollah has denied involvement in the attack.
Judge Royce Lamberth, who handed down the ruling on Friday, said it was a warning that attacks on US citizens will not be tolerated.
He said the award "may be the largest ever entered by a court of the United States against a foreign nation".
The Iranian government dismissed that ruling, saying the decision was "provoked by the Zionists".
US troops were deployed in Lebanon in 1983 as part of a UN-sponsored multinational peacekeeping force during the Lebanon's civil war.
Wave of attacks
On October 23 of that year, an explosives-laden truck rammed through barricades and detonated in front of the US barracks in Beirut, demolishing the building.
The attack was the "most deadly state-sponsored terrorist attack made against American citizens" until the September 11 attack on the World Trade Centre in 2001, the judge said.
As part of the same wave of attacks, a French barracks was also bombed, killing 58 French soldiers.
In his ruling from a federal court in Washington DC, Lamberth wrote that he hoped his judgment "will serve to aid in the healing process for these plaintiffs, and simultaneously sound an alarm to the defendants".
But the families will find it hard to collect the money, which they hope to secure through the seizure of Iranian assets around the world.
A spokesman for the families said they were lobbying the US Congress for legislation that would make it easier to chase down and seize Iranian assets.
Iran denies responsibility for the bombing, although it was instrumental in the founding of Hezbollah in the 1980s.
Lamberth issued his decision after considering claims by 1,000 family members and a small number of survivors.