I think this is what they are referring to: https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge.
TL;DR: The bridge collapsed because soldiers marching on it created force they hadn't anticipated, soldiers breaking step supposedly don't have as much of an impact.
I think this is what they are referring to: https://en.wikipedia.org/wiki/Broughton_Suspension_Bridge.
TL;DR: The bridge collapsed because soldiers marching on it created force they hadn't anticipated, soldiers breaking step supposedly don't have as much of an impact.