The Battle of Bosworth more or less fits the bill, except I wouldn't call Henry Tudor a hero.

In 1485, Richard III had been King of England for two years. It was the tail end of the Wars of the Roses, an incredibly messy and long-drawn-out struggle for the throne of England between the rival factions of York (Richard's house) and Lancaster. The Lancastrians had landed on a guy called Henry Tudor as their claimant. Henry had been living in France, and in August of 1485 he gathered an army of Lancastrian exiles and French mercenaries and sailed for England to challenge Richard.

Richard had warning of this, and got his forces ready. He couldn't just command every able-bodied man in England to fight for him, though. He could raise armies from his own lands, but apart from that, he was dependent on nobles, each of whom could call up his own men and join Richard's forces if he felt like it - out of loyalty, or because Richard had offered him something in exchange, or whatever.

The nobles who matter here are the Stanleys. The Stanleys had been kind of patchy about whether they were Yorkists or Lancastrians, and while William Stanley was a Yorkist, his brother Thomas had a long history of having a strained relationship with Richard - especially since he'd become Henry Tudor's stepfather. Stanley also had a reputation for not deciding which side he was on till the last possible minute. He hadn't joined a previous revolt in Henry's favour, though, so Richard had hopes that he would stay loyal in this one.

The two armies met somewhere near Ambion Hill. Richard had between 7500 and 12000 men, depending on whose estimate you take; Henry had between 5000 and 8000, leaving him solidly outnumbered; and the Stanleys, with between 4000 and 6000 men, were hanging around on the sidelines watching the battle till they decided which side to take.

Richard led his army into battle. Henry was much more modern about it: he kept himself safely behind his forces, with his bodyguard, and didn't take part in the fighting. At a certain point in the fighting, Richard spotted Henry and made for him, with a bunch of his men in tow. Henry dismounted and hid among his bodyguard.

That's when the Stanleys finally decided to join in. Seeing Richard separated from his main army, they went for him, drove him back from Henry, knocked him off his horse (hence 'A horse, a horse, my kingdom for a horse!') and killed him. And Henry became King Henry VII, founder of the Tudor dynasty.

It's by no means certain that Richard would have won the battle if the Stanleys had stayed out of it - the Lancastrians were putting up a hell of a fight. But he did have the numbers on his side, and he was practically within touching distance of Henry - who not only was on foot while Richard was mounted, he wasn't much of a fighter and wouldn't have stood a chance in hand-to-hand combat with an experienced soldier like Richard. So, from the Lancastrian standpoint, reinforcements did swoop in just in time to save the day.

Henry saw it this way, as reflected in the account of the battle by his official historian, Polydore Vergil. Vergil says that Stanley found Richard's crown (not a big ornate thing, just a circlet that would have gone over his helmet) in a hawthorn bush after the battle, and put it on Henry's head. It's unlikely that things literally happened this way, but the story is shaped to symbolically express that Stanley was the one who made Henry king.