Explanation: During Caesar's Gallic Wars, some Germanic tribes thought it would be a good lark to cross the Rhine and do some raiding. Caesar, rather than fording the river or crossing in boats, built a bridge in ten days, crossed 40,000 Roman soldiers over, did some retributive raiding of his own, then crossed the bridge back to Gaul... and burned it so he couldn't be followed.
This was stunning to the Germanic tribes, as they had thought themselves safe on the far side of the Rhine from large-scale intervention - bridges of the kind and scale Caesar and the Romans built were unknown to the Germanic tribes, and they watched this architectural marvel be built in a matter of days - and then be burned once Caesar was done with it. Quite a shock!