In case you were wondering, if soldiers didn’t break step while crossing a bridge, the vibration of the rhythmic step and swing of their arms might put too much stress on various parts of the bridge.
The Book of Knowledge: The Children’s Encyclopedia. E.V. McLoughlin, ed. The Grolier Society Inc., New York. 1954.
Stay tuned for more on wonder as soon as I catch up. It’s a wonder-full topic.