I imagine that most newcomers to the galactic stage usually take a few years, if not decades, before they even begin to think about adopting some form of directed-energy weapon as standard issue. Earth at the time of First Contact certainly didn't have the industrial base to support DEW weapons, certainly not in the numbers necessary to arm the armed forces of any major nation. And it's traditionally been a bad idea to make your military's logistics dependent on the good graces of another nation who might come under pressure to stop filling your weapon orders.

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CdrMike, Overwatch Reject