The event goes like this;
A Muslim woman was captured by the Romans and imprisoned, so the Romans laughed at her. The leader of the Roman king sarcastically said, "You won't ever leave this place until the muslim king himself removes you from these shackles." The woman shouted"Wa Mu'tasima!" (Oh my grief, Mu'tasima!).
The Roman King laughed at her and said : "He will never come to save you unless he was riding "ablaq" (a kind of horse that is colored with black and white). (he said this in a way of mocking her and showing the impossibility of the situation.)
A Muslim man who was in the land of the Romans heard the woman call out the Muslim kings name, he found out she was Muslim. So he ran to the Muslims and told them about what he had heard.
When Al Mu'tasim heard of the news, he gathered a whole army, and said the famous quote "When the first person of my army reaches the land of the Romans, the last of my army will have left us (i.e. the Muslim barracks)." He made the entire armyl ride the"ablaq" horses. So they set off, and he went with them on his black and white horse too.
When they reached there, he took control of the area, captured it and himself entered the prison where the Muslim woman had been imprisoned.
"Who are you?" she said.
"I am al-Mu'tasim." He freed her from her shackles, a sign of humiliation for the Romans.
Al Mu'tasim took over the land where this occurred, because they had waged war against the believers by humiliating a Muslim woman. Just like Allah's Messenger expelled the Banu Qaynuqa, for humiliating a Muslim woman.
http://salaf-stories...a-mutasima.html
