TEHRAN, Iran — Baku, the capital of Azerbaijan, hosted the presidents of Iran and Russia on Aug. 8 for the first trilateral summit of the three countries. The three leaders reached agreements on a range of issues, from efforts to pave the way for the development of the North-South Transit Corridor, to coordinating their fight against terrorism and extremism.

On the bilateral level, Iranian President Hassan Rouhani’s visit to Azerbaijan could also be seen as the beginning of a new era in Baku-Tehran relations. Indeed, among other developments, the Azerbaijani government agreed for the first time to facilitate entry visa issuance for Iranian citizens.

If the achievements during the Baku summit are put in the context of Iran’s broader ties with its northern neighborhood, it appears that relations are entering an unprecedented phase. For instance, earlier this year the Georgian government removed visa requirements for Iranian citizens — restrictions that strikingly were imposed just years prior. Indeed, Iran and Georgia have in past years been engaged in serious negotiations to expand bilateral cooperation, especially in the energy sector. Armenia, another South Caucasus country, has also experienced a thaw in its relations with Iran. On Aug. 13, it was announced that the visa waiver agreement between Tehran and Yerevan has entered the implementation phase. To its northeast, Iran’s relations with the Central Asian republics have also been considerably growing in past years. The latter has been symbolized in the 2014 inauguration of the railroad connecting Iran with Turkmenistan and Kazakhstan.

Iran’s diplomatic success in the Caucasus and Central Asia comes at a time when the situation in another neighboring region, namely the Arab Middle East and the Persian Gulf, is growing increasingly challenging for Tehran. Not only have past tensions in this region continued, but in some important cases — including ties between Iran and Saudi Arabia — there has been an unrelenting escalation of hostility. Thus, why has Iran been able to secure unprecedented diplomatic achievements in the South Caucasus and Central Asia but not in the Arab Middle East? There are five main answers to this important question.