Err, name three. I'm willing to be instructed, but it is the first I've heard of it. And I know rather a lot of Roman history.
Maybe he'll cite the
Teutoburger Wald :-)
I suspect he might be thinking of some of the skirmishes during the Gallic Wars in which some numbers of the Gauls would slip away rather than face the Romans, but I can't think of any major battles in which the Romans won by intimidation.
I'll name a few, there are many, many from which to choose. The most famous, that pretty much everyone who has looked at Roman history knows and no doubt you may have heard of, was Octavian advancing on Antony, Antony's troops and allies melting away and ending the wars of the triumvirate. A second was Vitellius' troops deserting upon the approach of Vespasian, as recorded by Tacitus. A third was Caesar advancing in Britain against Cassivellaunus, as recorded in De Bello Gallico by Caesar. A few skirmishes were enough to deprive Cassivellaunus of his allies and make him sue for peace.
As I said, an excellent strategy when you have the goods to back it up.