Every time a sufficiently powerful military breakthrough arrives there are attempts to ban it, or declare using it "dishonorable", or whatever the equivalent is.
Consider chemical warfare in WWI vs. chemical warfare in WWII. I'm no military historian, but my impression is that it was used because it was effective, people realized that it was lose-lose relative to not using chemical warfare, and then it wasn't used in WWII, because both sides reasonably expected that if they started using it, then the other side would as well.
One possibility is that this only works for technologies that are helpful but not transformative. An international campaign to halt the use of guns in warfare would not get very far (like you point out), and it is possible that autonomous military AI is closer to guns than it is chemical warfare.
Chemical warfare was only effective the first couple times it was used, i.e., before people invented the gas mask.
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