Each branch of the military has a different way to show their unit pride. U.S. Army soldiers wear easily identifiable patches (a shoulder sleeve insignia) on the left shoulder of their combat uniform.
The SSI shows the current duty station that the soldier is attached to. If the soldier has deployed to a designated combat zone, they can also slap that unit patch onto the right shoulder to wear for the rest of their career.
This leads to a little game soldiers play, reminiscent of kids playing with trading cards, where they trade unit patches with one another or leave one with the Bangor Troop Greeters.
Depending on the unit, this may be for regiment, brigade, or division — with the patch being from the highest distinct echelon (so if you were in the 101st Airborne, you would wear the 101st patch and not the XVIII Airborne Corps patch).
The patch was conceived to inspire unit pride and to identify other soldiers in the unit. The first to adopt a shoulder patch was the 81st Infantry Division in 1918. When they deployed to France shortly after adopting it, their patch drew much disdain from other units in the American Expeditionary Force.
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The "Wildcat" Division's unit patch was brought to the attention of Gen. John J. "Blackjack" Pershing by a fellow officer because it was unbecoming of the uniform. After some consideration, the only American to be promoted to General of the Armies in his lifetime decided that the 81st should keep their unit patch and suggested other divisions to follow suit. The patch became officially recognized on Oct. 19, 1918, and many more followed shortly after.
Ever since then, soldiers have a treasured relationship with their unit patches (and even more if they deployed with them.) Through their patch, they stand tall among their brothers in arms of the past — adding to their legacy.