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I think china investment in Iran can help china achieve domestic supply chain in all industry.Has China committed to BRI projects in Iran? Or has China dropped Iran immediately if asked to by the US pre Trump? For Iran, China has always been a lot of promises with little delivered. When Chinese companies were awarded contracts, they didn't develop the fields and appeared to delay. Of course China has also bought small amounts of Iranian oil during trump's maximum pressure campaign. And Iran has increasingly turned to Chinese cars and other industrial products. But if China wants good relations with Iran, it has to actually deliver the promised investments rather than just talk. And China did support UN sanctions on Iran at America's request. Iran wants more BRI, but will China give more?
Pakistan isn't failing because of competition from chabahar, which is not a significant port today. Pakistan has a failed political system and Pashtun terrorists. It's very corrupt and partially controlled by the CIA. Pakistan could have been like Bangladesh, they only have themselves to blame for failing in that
Most of the Arab world doesn't care about Iran. Some Arab countries like Algeria, Iraq, Syria, Lebanon, the Houthi government in Yemen and Qatar have pretty good relations with Iran. Iran has a lot more sympathy in Arab populations for its support for the Palestinian struggle for freedom. Iran has friendly ties with Turkmenistan and Pakistan and some influence in Afghanistan.
Saudi Arabia, Turkey and Iran are all struggling for influence in the region. Supporting Saudi Arabia and their partners Egypt and the UAE won't help in getting more market access in the wider Arab world and risks supporting Saudi Arabia's export of Wahhabi extremism. But Saudi Arabia and Iran have no territorial disputes. They don't need to be enemies if the US isn't in the region anymore. China should promote trade and peace between them rather than following the US strategy of sowing division.