They were only going for a parade in Tajikistan, but even that disturbed Uzbekistan, which refused to allow the plane carrying them to overfly Uzbekistan.
The troops eventually got to Tajikistan by flying over Afghanistan-where the airspace is controlled by the United States!
The Tajik daily Najot, affiliated with an opposition party, said Iran sent 150 troops who marched in the country’s Independence Day parade September 9. It was the 20th anniversary of the country’s separation from the Soviet Union.
The Tajik government spoke of the event as more than a mere 
ceremonial appearance. Tajik Defense Minister Sherali Khayrulloyev said, “Friends and enemies once again saw that, if needed, the Iranian armed forces will be in Tajikistan in two hours. If necessary, we will go to Iran. We will support each other.”
The Iranian ambassador to Tajikistan, Ali Asghar Sherdust, said the Iranian participation was arranged on very short notice. He said the Tajik president suggested the visit to the embassy on September 4, just five days before the parade. The Tajiks sent letters to Supreme Leader Ali Khamenehi and President Ahmadi-nejad the next day. The ambassador said the Iranian leadership approved the deployment within two hours.
The ambassador told the daily that Uzbekistan refused Iranian requests for overflight permission. There were reports that Turkmenistan had also denied overflight rights, but the ambassador said Turkmenistan was never asked since Uzbekistan was not cooperating.
With that route gone, the only route available was across Afghanistan, where the United States Air Force effectively controls the airspace because it flies combat missions there every day. The ambassador said the Afghan government approved the overflight. He said nothing about the contact the flight would have had with American air controllers.
The last time any Iranian military units served abroad was during the 1970s when Iranian troops fought rebels in Oman on behalf of the sultan there.
The daily speculated that the Tajik government was cozying up to Iran because it was seeking a “powerful protector” to guard Tajikistan from any possible Russian takeover. While Russia likes to throw its weight around the former Soviet republics, there has not been any indication of any interest in seizing the country militarily. Nor is it likely that Iran would be willing to insert itself between Tajikistan and Russia in any dispute.
The Tajik language is a variant of Farsi and the two countries have many cultural affinities because of that.