Union suits, better known as “long underwear” popular in the early 19th century evolved into what we now know as the “T-shirt.” This was achieved simply by separating the top & bottom halves.
During WWI European troops wore comfortable, lightweight T-shirt during the hot summer months. American troops wore heavy wool uniforms. Needless to say, once WWII rolled around, T-shirts became standard issue for the Navy & Army.
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Source: classB