War is Waged and al-Mamoon Wins

The gap between the brothers became wider, and the presentiments of the tragedy to befall the two brothers were in sight when al-Amin announced in Baghdad his decision to drop the name of his brother al-Mamoon from Friday sermons and substitute it with that of his own son Mousa whom he named his successor, and he sent letters to places far and wide in this meaning. Al-Mamoon rose to defend his right and started planning to overrun Baghdad, the capital of the government, while al-Amin was gathering troops to take over his brother's domains. Both armies finally clashed and fierce battles ensued in more than one location, and in the end al-Mamoon came out victorious, took control of Baghdad and killed al-Amin. All of that became possible due to the planning of al-Fadl ibn Sahl, who was nicknamed "Dhul-Riyasatain," i.e., the man who had a say in two states, and his brother al-Hassan, assisted by an elite group of military experts and top political advisors. 

Having won victory over his brother, al-Mamoon tried to make Marw the base of power for the Abbaside dynasty instead of Baghdad due to the advice of his army leaders and top political aides who were credited with regaining his right to the caliphate after al-Amin had deposed him, and because of his own feeling of gratitude towards the city that assisted him and brought him victory during the darkest periods of his political crisis.

Adapted from: "Imam al-Ridha (a.s.), A Historical and Biographical Research" by: "Muhammad Jawad Fadlallah"