Iran's influence in Afghanistan is set in concrete: new roads crisscross the country, power grids supply remote cities with electricity, and planned railways form ties that bind.
Tehran's also leaves its mark in less obvious ways, for example through its export of cultural and political views, strong media presence, and the funding of religious schools.
But even while welcoming the much-needed assistance, Kabul has always warily eyed Tehran's advances.
Now that caution has given way to tension, leading observers to warn that Tehran is poised to make Afghanistan an ideological battleground should Kabul not see things its way.
The tipping point, says Najib Mahmoud, professor of political science at Kabul University, is the recent signing of a long-term strategic agreement between Afghanistan and the United States.
"This agreement might make Iran feel like it is surrounded," Mahmoud says. "Secondly, if the U.S. maintains control in Afghanistan, considering the state of relations between Iran and the U.S., Tehran will feel that Afghanistan could be a threat in the future. And this will create tension between the two countries."
The agreement signed on May 1, while light on specifics, is intended to signify the United States' financial and security commitment to Afghanistan through 2024. On the military level, the security pact sets general terms for the funding and maintenance of a large Afghan National Army.
Although not yet determined, the U.S. commitment could entail a small contingent of troops staying in Afghanistan beyond 2014, when foreign combat troops are scheduled to leave. The remaining U.S. troops would assist Afghan forces in defending Kabul's sovereignty, including taking part in actual combat missions against external threats.
Mahmoud says the prospect of a large Afghan military backed by the U.S. military and Western financial clout is a source of deep concern for Iran.
Tehran, he explains, fears that an extended U.S. military presence in Afghanistan would provide Washington a strategic advantage to conduct surveillance and perhaps even future military attacks on Iran.
Tensions Rise, Accusations Fly
In the wake of the signing, Kabul-Tehran relations have soured considerably.
Last week, Afghan officials expressed outrage after Iran's newly appointed ambassador to Kabul reportedly demanded Afghan lawmakers reject the U.S.-Afghan agreement, which has to be ratified by the Afghan Senate and parliament before it can go into effect.
The diplomat, Abul Fazal Zahrawand, also reportedly threatened to expel all Afghan refugees -- estimated to number around 1 million -- from Iran if Afghan officials failed to heed his demands. Afghan lawmakers responded by publicly accusing Tehran of meddling in Afghanistan's domestic affairs.
As the diplomatic row played out, Afghan intelligence leaked a video purporting to show two Afghan men confessing to spying for Iran
and attempting to carry out terrorist attacks in Afghanistan. They admitted to belonging to Sipah-e Mohammad, a group of Afghan refugees who allegedly received training by Iran's Islamic Revolutionary Guards Corps.
The Revolutionary Guards, the men claimed, were recruiting and providing training for Afghan militants from the Taliban movement and the extremist Hizb-e Islami group in training camps inside Iran.
Adding to the growing discord, Afghan intelligence announced that allegations that up to 40 Afghan members of parliament were on Tehran's payroll were being investigated. And Iranian-funded media outlets also came under scrutiny for alleged incitement of anti-American and antigovernment sentiment.
A Battleground, Once Again
Afghan political commentator Wahid Muzhda says that the further escalation of tensions could have grave consequences for Kabul. The worst-case scenario, he explains, could even see Afghanistan once again become the battleground for a regional proxy war.
After the defeat of the Soviet Union in 1989 and the collapse of the subsequent regime in Kabul in the early1990s, Afghanistan's neighbors funded, armed, and trained their Afghan proxies to gain regional leverage -- a move that fuelled the country's descent into civil war.
Iran provided assistance to Afghan Shi'ite and Persian-speaking groups, while Pakistan, with the aid of the U.S. Central Intelligence Agency, armed Sunni Islamists in Pakistan who later gained control of Afghanistan.
Muzhda says it's "natural" the Kabul's U.S. pact would "worry" its neighbors. "When this deal was signed, without doubt, proxy war has looked like more of a possibility. Why? Because Iran and the U.S. will probably not fight directly," he adds. "Afghanistan is the best place for them to put pressure on the U.S. This will make Iran's opposition to the Afghan government and the U.S. much clearer."
Although Muzhda notes there is evidence that neighboring countries such as Iran are interfering in its domestic affairs, he says Kabul may have to pull its punches.
This is because Afghanistan depends heavily on its larger and more powerful neighbors for energy supplies and trade, in addition to its stability and security.
"Our trade routes go through our neighbors. If Pakistan and Iran close their borders for a week, even if they are not at war against us, you can imagine what would happen," Muzhda says. "It is necessary for us to have good relations with our neighbors."