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Robotic dog-mounted rifles are now a thing thanks to US Army
(interestingengineering.com)
This is a most excellent place for technology news and articles.
No, that assumes that resources and energy are unlimited. This is the main problem with any "grey goo" scenario.
And it wasn't people being killed that stopped WW2. People are killed in every war. The defensive weaponry was more powerful than the offensive weaponry. Machine guns were only used on defense because they were heavy. Also, artillery was not mechanized so it was hard to move everything forward quickly.