As the Arabs expanded beyond the Arabian Peninsula after the coming of Islam, they encountered two great but fading empires. To the north, they met the Byzantines, the remains of the Roman Empire that had been destroyed in mainland Europe. To the east, they met the Sassanians, a vast sprawling empire centred around modern-day Iran and Iraq. Within a matter of years, the Arabs came to rule most of the territories and peoples of both empires.
One interpretation of history has it that the Persians never forgot this defeat. That throughout the long centuries of Arab rule, they chafed, staging rebellions, attempting to throw off Arab rule and refusing the language, culture and laws of the succeeding Arab civilisations.
That, certainly, is true in part. Persians rightly considered theirs a civilisation as well as an empire, with its own bureaucracy, standing army and politics. Their language had a proud history stretching back more than a millennium before the conquest by the Arabs.
Yet as with most stories that seek to explain modern-day political rivalry with reference to faded history, it is selective.
Many Persians in the Sassanian empire were intrigued by the religion and culture of Islam. It was dynamic and its adherents had a messianic zeal that found a ready audience. Islam promised a new type of relationship between man and God and the politics of the nascent Islamic empire similarly heralded a new way of interacting between rulers and the ruled. The Sassanian empire, by contrast, was several hundred years old at that point and riven with internecine fighting.
In any case, the small Arab armies could not have ruled such vast territories east and west without the implicit consent, or at least tacit acceptance, of the people. For hundreds of years after, the world that the Arab Muslims ruled was neither predominantly Arab nor Muslim.
In time, Arabic became the official language of Persia, although Persian never vanished - important literature was written by Persians in Arabic. The institutional knowledge of the bureaucrats of the Persian lands helped the Arabs in their empires, east and west. To imagine that there was a clear distinction between Arabs and Persians is to profoundly misread how the two peoples interacted over centuries.
Fast forward to today and such cooperation seems impossible.
Behind every conflict raging in the Arab world appears to be the hand of Iran. In Lebanon, the Iranians are supporting Hizbollah as a state-within-a-state. In Yemen, the Iranians provided assistance to the Houthi rebellion in the north. In the two worst conflicts in the Arab world - in Iraq and Syria - Iran is a pivotal player. The raging sectarian battles that have engulfed Iraq were sparked by the US invasion - but the fuel has come in large part from Iran.
In Syria, Iran's involvement is essential to the Assad regime's ability to survive and continue slaughtering its people. If one views the Arab world today, the most persistent meddler is not America or the West, but Iran.
Small wonder then that a leaked diplomatic cable from 2008 quoted a senior Saudi leader urging the United States to attack Iran and "cut off the head of the snake".
Amid such acrimony and history, is it even possible for Hassan Rouhani's presidency to create change? When he took the oath of office on Sunday, Mr Rouhani vowed that he would bring stability to a turbulent region. But the winds that fan the flames of regional wars so often blow in from Iran.
Mr Rouhani has been making conciliatory noises towards the US. But he would do well to realise that he needs to build bridges in his own backyard first.
And that need to understand the importance of neighbours cuts both ways.
A secure and prosperous Iran is in the interests of both the Arabs and Iranians. Despite all the problems that preceding Iranian governments have brought to Arab lands, there is still ample reason to imagine that this can be overcome. Because Iran is a vital country for the stability of the Middle East, it is not enough for the Arabs to contain it; they must embrace it.
Iran remains the most dominant country on the eastern edge of the Arab world. Iran has religious leadership in the Arab world, with many of the main seats of Shia scholarship. And everywhere from Iraq all the way down to Yemen, there are ties of family, language and business.
Bringing Iran back into the fold is, therefore, an important task for regional leaders. But in order for that to happen, Iran needs to adjust its perception of who is a threat and who is an ally.
Despite mistrust - and unhelpful stereotyping - there is no shortage of Arab leaders willing to offer Iran the hand of friendship. But it must be willing to take it. The country's leadership must be willing to make hard choices that will reintegrate it into the region.
In time, Iran's enmity with America will fade. The United States is leaving the Middle East on its long pivot east. As it does so, its power and influence in the region will fade and new alliances will replace it. But as long as there is an Iran, it will be bound by ties of history, blood and land to the Arab world.
Iran's future stability and prosperity are not dependent merely on rapprochement with the Americans - they are dependent on repairing relations with the Arabs.
On Twitter: @FaisalAlYafai