Is it really necessary to break step when marching across a suspension bridge?

Break step is a term used when marching to not march in step.

The urban legend is that if you march across a bridge and every soldier is in step, if will / can create the same harmonics as the bridge and cause it to collapse.

Is this an urban legend?

There is a somewhat famous case of a footbridge (the London Millenium bridge, opened in 2000) that swayed violently (though didn't coillapse) when people started walking acrosss it. The reason being that, although it was built on the assumption that the sway caused by one person's step would be canceled out by the sway in another direction caused by another persons step (given a large enough number of people with random strides), but it was discovered that a slight swing would cause people to adjust their step in a way that, while it improved their balance, amplified the sway.

I would imagine a weak enough bridge could be torn apart from the lateral forces, but most bridges, I would imagine, would hold up.

There is a story about a bridge which did collapse from something similar - wind - the Tacoma Narrows Bridge

The fear is based on harmonics, in the case of the Narrows bridge the wind set up a dangerous harmonic (wave pattern) which intensified the swaying and the waving pattern of the bridge until the stress of bending and swaying surpassed the strength of the material, the bridge broke and collapsed due to the intense wave form.

Yes it is possible that an army marching in step could set up a dangerous harmonic, but there is a point where the swaying or waveform would be strong enough to throw the soldiers out of step, or worse, off the bridge - this is assumed to be way before the bridge would break. Once the militia are thrown off step (forced to break step) the bridge would start slowing its motion and settle back down.

Break step is used in suspension bridges to help prevent the swaying from starting - it will in many cases - but there is no fear of bridge failure, just wisdom to make it across faster and easier without too many sea sick troops to deal with.

Cheers

David