Wearing an army or marine tartan

[KnittedReenactor]

The US Army, USMC, USAF, and USN tartans are unofficial and not accepted by any of the military branches. They are tribute or commemorative tartans and free for anyone to wear.
As long as you are not trying to pass yourself off as current or former service that you did not actually do, you should not have any problems. Most people in the wider world won't recognize the pattern, and those who do typically won't get huffy.

As for upsetting others... whatever you do or say, if someone isn't offended, then you aren't doing it right.