Soldiers have long modified combat boots for an extra bit of comfort because the boot sole and interior are not the most comfortable in the world. During Vietnam war, an experimental "barefoot boot" was developed, it was used by MACV / SOG (Military Assistance Command, Vietnam – Studies and Observations) because Communist trackers would come running whenever they discovered U.S. jungle boot tracks.
If you are going to look closer they even took care of the smaller size of the feet of the Vietnamese which meant that the boots cannot support the heel causing discomforts to the US soldiers. In addition to this plan, MACV SOG dropped over 20,000 pairs of US jungle boots over the Ho Chin Minh trails so that the North Vietnamese would wear them instead of their tire rubber sandals.
Why did the US do that?
It's simply because the North Vietnamese would leave the same footprints as the US soldiers. If they saw similar boot prints being left, they were not left by their allies because there were none around.
Why wouldn't the SOG just wear tire sandals to blend with the Vietnamese?
US soldiers were not used to wearing sandals and it would take time for them to properly adapt and harden the skin and develop calluses. It will also degrade their performance, safety and health due to blisters, unsure footing and their feet will be exposed to moisture. Keep in mind that there is no harm in ensuring to have good feet, as they are the primary means of transportation.
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Reference:
https://forum.birthdayshoes.com/discussion/3482/barefoot-kind-of-shoes-in-vietnam-war