OTL... the two have no major friction and friendly relations. Surprisingly I've never seen any AH or technothriller works exploring a hypothetical conflict between these two nations.
So let's discuss what a war (sometime between the dissolution of USSR in 1991 and the present) between Iran & Turkmenistan could look like.
Maybe….
The Prelude:
Iranian Major Ghazi was sickened by what he saw- The Border Guard vehicles had been ripped apart. Walking the ambush site, he identified the terrorist's former positions. The ambush had been sophisticated and the terrorists were using the latest in both anti tank and small arms weaponry. A talk with a dazed survivor revealed that the terrorist weapons included an ATGM, new model RPGs and crew served GPMGs.
Endless acronymns aside, this was far different than the customary spray and pray Ak-47 ambushes committed by Turkic narco gangs thinly disguised as ethnic separatists. Intelligence increasingly supported a conclusion that the weapons and training originated in Turkmenistan.
Going from bad to worse:
Iran seemed to be dwindling by the day. Oil prices were collapsing. Iranian oil support a large population- almost all of whom were citizens. No, creative exclusionary citizenship requirements in Iran. Then adding insult to injury, once low level bandit activity had exploded into full insurgencies in Iranian Kurdistan, Baluchi, and Turkmen areas. This, despite the fact that the Iranians had shown restraint regarding casualties for years.
Escalating ambushes gave way to Turkic seperatists taking over entire towns and creating alternative governments- east Ukraine style. Major Ghazi suspected- no knew, that the sophisticated communications gear being used came by way of Ankara to Turkmenistan to the insurgents.
Ghazi, now a Lieutenant Colonel, also knew that the Iranian commander's conference was going south. Too many Revolutionary Guard commanders supplemented by militia leaders of little training were mixing Shia theological references with terms like "disinfecting" and arranging for copious quantities of fire support from "Divarty assets" (Ghazi found it interesting that clinical American military acronyms were ohh so trendy in Iran).
"Hearts and minds" was not mentioned once. Ghazi knew that these were the wrong troops with the wrong mentality- but Iran simply did not have the right troops. Especially not with three insurgencies raging and a collapsed economy.
The War
A "Motley Crue" of Iranian regulars, Revolutionary Guardsmen, sanctioned vigilantes and tolerated criminal gangs moved into all three of Iran's resistive areas. "Disinfection" and "Fire Support" combined together to create destruction and columns of refugees- then, in some areas, ethnic cleansing. The Kurds and Baluchis lacked support from nation states. The Turkmen, however, did not.
Alarmed at reports of ethnic cleansing, Turkemenistan delivers a message to the Tehran: - Withdraw the entire Motely Crue completely from Turkic areas immediately or face war.
Not even the fact that Turkey also quickly affirmed Turkemistan's demands cooled Iranian anger. Though the US could create Kosovo, the collective Turks were not the USA- not even close. Any Iranian withdraw would lead directly to loss of sovergnity.
The Iranians double down....