Any history book will be filled with such stories. Depending on the outlook, I'd say all history is like that.
Take any one event. Let's pick any decisive moment in history. Say, the battle of Salamis. Now flip it to the side of the Persians and you have the kind of blunder you're looking for.
Very true when talking historical events. Say the USA lost the American Revolution and it's now a land mass of Brits that can't believe how foolish the revolutionaries were. (Although if other colonies are any indication independence may have eventually happened anyways)