When a troop crosses a suspension bridge, the soldiers are asked to break their steps. Why?

When a troop crosses a suspension bridge, the soldiers are asked to break their step, because when the soldiers march in steps, each soldier exerts a periodic force in same phase and therefore the bridge executes the forced vibrations of frequency equal to the frequency of their steps.

Now, if the frequency of the steps becomes equal to the natural frequency of the bridge, the bridge will vibrate with large amplitude due to resonance and hence, the suspension bridge may collapse.