Julius Caesar's Greatest Military Victory [video]
youtube.com
youtube.com
But, I'd argue that Caesar's crossing of the Rhine was his most impressive victory. He built a bridge over an impassible river, marched his army across, walked around a bit, crossed back over and dismantled the bridge.
So yes, corn existed in Europe long before Columbus.
Romans would not have gone out for corn, though. They had the word 'granum', from which we derived 'grain'.
The use of the word "corn" to refer exclusively to maize and not other cereal grains is a quirk of North American English that apparently has also spread to Oceania, but is not shared by much of the rest of the English-speaking world.
The Roman army was very adept and building, and then tearing down, temporary fortifications.