Re: Unity among Muslim (Shia/Sunni) brethrens puts fear in the hearts of the oppressors.
After the Iranian revolution, Imam Khomeini made a historic ruling for the Shia Muslims allowing them to pray behind Sunni Imams at the local Masajid if needed, prior to that such prayers were not considered valid in the eyes of Shia Scholars.
This was certainly a very significant ruling for Islamic Unity.