Tehran, Iran - It has been a year since Iran shot down a US Global Hawk surveillance drone over the Strait of Hormuz.
Tensions have escalated since and the two countries came the closest they have in recent years to all-out conflict in January, when the United States assassinated one of Iran's most popular leaders - Quds Force commander, Major General Qassem Soleimani.More: Are the US and Iran on the path to war? What does Iran want in Afghanistan? Why is the Strait of Hormuz so strategically important?
Iran retaliated by firing missiles on a base used by US soldiers outside Baghdad, injuring dozens of American troops. Both sides took a step back but have continued to engage in hostile rhetoric, while the US recently announced new sanctions against Iranian shipping companies.
But behind the scenes, throughout ongoing tensions, the two countries have continued to cooperate - albeit indirectly - on relief and reconstruction work in Afghanistan.Chabahar port
Chabahar port in the Sistan-Balochistan province of Iran is a hub of activity and one of the country's economic lifelines. Located on the southeastern edge of the country, it is the only Iranian entity of its kind that has been spared from American sanctions.
A US State Department spokesperson told Al Jazeera: "After extensive consideration, in November 2018 the administration granted a narrow exception [...] to allow a limited number of activities that support the reconstruction and development of Afghanistan, a key US national security interest. The exception provided for the reconstruction and development of Afghanistan and allowed for the operation of Chabahar port in support of these goals."
As a result of the exception, Afghanistan is able to import Iranian fuel and Iranian-made goods considered vital for humanitarian assistance, but the port also serves other US interests in the region.
The spokesperson said that the president's South Asia strategy "underscores our ongoing support of Afghanistan's economic growth and development as well as our close partnership with India. We seek to build on our close relationships with both India and Afghanistan as we execute a policy of maximum pressure to change the Iranian regime's destabilising policies in the region and beyond."
Chabahar port, with road and rail links through Iran, offers a more secure alternative route to Gwadar port in Pakistan, operated by the Chinese.
It gives India - a key US ally in the region that is assisting with Afghan reconstruction - access to the landlocked, war-torn country. Other than moving goods and equipment bound for Afghanistan, for India, it is also a trade link to lucrative markets in Central Asia and beyond.
So, not only is an Iranian port helping the US rebuild Afghanistan, but it is also helping India act as a counterweight to Chinese and Pakistani influence in the region. Chabahar port's strategic location makes it a kind of political Bermuda triangle, where the normal rules of the US's hostile Iran policy do not seem to apply.
"Iran's influence in the region is not ignorable," said Mohammad-Hussein Ansarifard, a Tehran-based political analyst.
"Iran can influence the whole security apparatus in the region. Iran's influence for example in Syria, Iraq, Lebanon, even in Yemen, is very, very big and very, I think, essential. [...] So, if Iran does not collaborate with Americans [...] then the problem in the region would be much more complicated and much more costly for the Americans. [...] I think this is a very good reason to let Iran have the Chabahar [port]."
During a cabinet session in May, President Hassan Rouhani described the port as a vital part of shaping Iran's economic future.
"One of our major projects in Sistan-Baluchestan province was reviving and using the port of Chabahar," he said, pledging to continue development work. "Sistan-Baluchestan is an important province to our country. It is sensitive in regards to our national security. It is said that Chabahar and Sistan-Baluchestan are the forefront of Iran's national security and that is very true."
India is currently the only other major investor in developing the port, but Iranian leaders have repeatedly expressed a desire to involve as many neighbouring countries as possible, to make the port a regional hub. The more countries investing in the port, the more difficult it would be to sanction in the future, according to one university researcher.
Seeing it up-close, Asarifard said, it becomes clear the port is more than just a transit point and has enormous potential.
"I think it's bilateral trade, not only between Iran and Afghanistan and so on," he said, adding, "It would even be a rest port for [ships] coming from Eastern Asia, going to Europe. They can stop in Chabahar for refuelling and for taking up new goods. I think it would be a connection - a very essential connection - between trade in the East and trade in the West."
He also said the port could lead to cooperation between the US and Iran on other regional security problems.
"Both now realise that the quarrel between the two countries weakens both of them and both are paying heavily for it," Ansarifard said.Old wounds
Recent US-Iran prisoner exchanges were seen as a significant step in the same direction.
And even though there are converging interests in Afghanistan - both want stability and to see US troops leave the region - future cooperation will require working past tensions, which in some cases have been simmering for decades.
The old US embassy in Tehran was renamed the den of espionage museum. For decades, it has been occupied by Iran's government, its boundary walls a canvas for anti-American artwork. The US flag is flown at half-mast and upside down, the signal for distress.
A security person looks on at oil docks at the Chabahar port [File: Reuters]
It is an ever-present reminder of old wounds and the broken relationship between the US and the Islamic Republic of Iran. But despite decades of animosity, and no direct diplomatic representation in each other's countries, there have been examples of quiet cooperation.
After 9/11, Iran was instrumental in helping the US fight the Taliban armed group, topple them from power and set up a new government in Kabul.
A 2010 report published by the Combatting Terrorism Center at West Point said Iranian influence in Afghanistan is inevitable and Tehran could - in equal measure - help or hinder US forces. Some Iranian experts hope Chabahar port can become a model for more future cooperation.