For those who have watched this week's episode of Sengoku Basara you may have found the flashback between Masamune and Kojuurou to be extremely cute and heartbreaking, you also may have been a little curious about that Hitotoribashi Battle they were talking about.
I was curious so I did my researches, and here is the result (because obviously, the Wiki article sucks ^^)
Let's sum up quickly to start.
Location : In the North of Japan, around the city of Nihonmatsu
Date : 1585
Opponents : Date (with 7 000 men) against Hatekeyama (30 000 men)
Everything started when Hatakeyama Yoshitsugu captured Date Terumune, Masamune's father.
Obviously Masamune wasn't happy at all at the idea, took some men and ran to his help, without much luck, even if they were victorious Terumune died during the fight.
The real battle of Hitotoribashi took place a little later, after the death of his father Masamune wasn't happy at all, so he and a few samurai decided to siege the castle of Nihonmatsu.
Despite the large discrepancy in the number of men in each side neither won.
One year later, Date's forces siege the castle once again, this time resulting in a victory for Date's side and the setting on fire of said castle by Hatakayama Yoshitsugu's son.