The $7 billion, 1,200-mile project will bolster Tehran's efforts to loosen the stranglehold of Western sanctions aimed at choking Iran's energy exports, the country's economic mainstay.
At the same time, it will reinforce energy-starved Pakistan's growing defiance of the United States and Washington's ongoing provocative unilateral use of missile-armed drone aircraft against suspected al-Qaida leadership cadres, airstrikes that violate Pakistani sovereignty.
The planned flow of 750 million cubic feet of gas from Iran's vast South Pars field in the Persian Gulf isn't expected to begin until the end of 2014 but the project inaugurated March 11 at the Iranian border town of Chabahar could be a winner for Pakistani President Asif Ali Zardari, who faces national election May 11.
The Pakistani leader, who's likely to run for re-election when his own five-year term expires in September, wants to be seen to be tackling his country's chronic energy shortages that cause daily power cuts and triggered a nationwide blackout in February.
Zardari says the much-delayed pipeline, first mooted in 1994, is all about economic survival.
"We've got to be economically sound and therefore this pipeline's a life pipeline as far as Pakistan's concerned," he said.
But it's also clear Zardari had to make some concrete gesture of Islamabad's growing dismay with the United States and the pipeline filled that bill, sending an unmistakable signal to Washington.
"Pakistan's long-running, ambivalent relationship with its principal financial backer and strategic ally has been more hate than love in recent times," observed international affairs analyst Simon Tisdall.
Anti-American sentiment has soared because of U.S. actions in Pakistan, including the May 2011 assassination of Osama bin Laden in his hideout not far from the Pakistani capital of Islamabad.
U.S. commentators have described the relationship as "toxic."
In this climate of distrust and dismay, expect Pakistan to continue to connect with Iran, even though Pakistan's largely Sunni Muslim and Iran's overwhelmingly Shiite. Given the growing hostility between the two sects, particularly Saudi Arabia and Iran, there may be considerable limitations on how far the relationship can go.
But one thing Pakistan and Iran have in common is their antipathy toward the United States, heightened by Zardari's efforts to loosen what Tisdall calls "Washington's suffocating embrace."
The administration of U.S. President Barack Obama has threatened that the pipeline project could breach Western sanctions imposed against Iran for its refusal to abandon its contentious nuclear program.
If the Americans follow through on that threat and impose sanctions on Pakistan as well, Islamabad can be expected to identify with Iran all the more.
Pakistan already imports 1 gigawatt of electricity from Iran and the countries are negotiating to more than double that.
The gas pipeline and the defiance of both Tehran and Islamabad in pursuing it "demonstrates the difficulty the U.S. faces in preventing Iran from leveraging its energy resources with its Asian neighbors," the International Institute for Strategic Studies in London observed.
Meantime, their common focus on China could indicate partnership in further energy projects.
China has been a key importer of Iranian oil and now Beijing's looking to Pakistan to bolster its energy conduits from the Middle East.
Zardari, ignoring the concerns of the United States and India, Pakistan's archrival, last month transferred management of the strategic port of Gwadar on Pakistan's southern coast from a Singapore company to Chinese control.
There are fears Beijing may seek to use Gwadar as a naval base in its military expansion contest with New Delhi, no friend of Pakistan, for control of the energy and shipping routes across the Indian Ocean, an arena for possible future conflict.
Beijing built the deep-water port in the first place as a key hub for oil and gas from the Persian Gulf, with plans to build pipelines overland to western China, rather than rely on maritime routes vulnerable to U.S. Navy interdiction.
The original concept of the gas pipeline from Iran to Pakistan included India, which is also facing severe energy shortages. But the Indians quit the project a couple of years ago, under U.S. pressure.
Beijing is even mulling having Iran extend the gas pipeline from Pakistan to China to satisfy Beijing's seemingly insatiable hunger for energy.