A battle that took place almost 70 years ago in Spain sets the stage for today's Geo Quiz. This battle was one of the last during Spain's Civil War in the 1930's.

It was a final attempt by Republican forces to defeat the Nationalist Army led by General Francisco Franco. The Republican side was fighting to keep an elected leftist government in charge of Spain. It had help from various international volunteer brigades.

Franco

That included Americans. They lost -- paving the way for Generalissimo Franco's long reign. Now, this pivotal battle took place along the banks of a river. This is -- in fact -- Spain's longest river.

Its source is in Cantabria -- not too far from Basque country. 565 miles later, the river empties through a delta into the Mediterranean.

So what's the name of Spain's longest river?

We'll give away the answer -- and tell you more about the battle here