Clearly, there is a lot of disagreement here.
For my part, I don’t get offended by civilians having and using a wide variety of military surplus or “military-style” gear. If you didn’t serve, then in my opinion you should not wear most of the uniform items, current issue or not (except for special circumstances, such as reenactments), but most military equipment, legally purchased at surplus stores, doesn’t offend me. Military equipment is sometimes the leading edge of technology in the field (and, sometimes, is really really far behind … just look at the miserable experience of early 20th century soldiers in extreme weather, especially cold weather), or is particularly well suited to certain roles. Backpacks are one of the areas where the military has been studying and implementing field-based changes to the equipment for decades. It makes sense to me to use that expertise, regardless of prior (or current) service status.
Another area where military tech has made huge advancements is in footwear. Military boots are much better than they were in the early 20th century, and are generally quite comfortable and useful for activities like hiking. I wouldn’t recommend buying a pair of Corcoran® Jump Boots unless you have a military requirement to have them, but most other military boots are really awesome for a wide variety of tasks.
Also, both base layer and outerwear are areas where the military has made (or inspired) significant advances. Both polypropylene long underwear as well as GORE-TEX® outerwear (such as is present in the Extended Cold Weather Clothing System) are top-notch wear, and it makes sense to me to take advantage of the military’s experience in this area, especially if you hunt, camp, or backpack in cold weather.
My only advice there would be 1) never, ever wear unit patches you didn’t serve with, and 2) don’t display any rank insignia. The rank insignia, in particular, are a calling card; in essence, you are saying “I was (or am) a (whatever rank insignia you display).” While I understand the argument that perhaps displaying them is merely a sign of esteem, I respectfully disagree. If you didn’t serve with the unit, don’t display the crest.
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