Why does the Army need a version of an Air Force weapon?
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Simple answer: because the AF is a not always available for Army missions. And its not wasteful: the GE has different specs and abilities.
The rest of your comment is just silly.
“BREAKING: Ukrainian forces have begun training on the HIMARS from the U.S.(Snr. U.S. defense official. US trainers working at an undisclosed location outside of the country. “