I've been very critical of claims of the dangers of AI, but this isn't about AI - this is about giving lethal weapons to autonomous systems.
And that's just stupid. All software has bugs, and even if you don't have a problem with autonomous killing systems - I certainly do - it's incredibly irresponsible to build systems that could kill just because someone left out a semicolon or couldn't convert between miles and km.
It's taking the "blue screen of death" experience just a little too literally.
https://en.wikipedia.org/wiki/List_of_coups_d%27%C3%A9tat_an...
Usually if a leader is assassinated, the assassin is a member of the country's military.
The US is comparatively rare in having had no coups since independence and only one civil war. The US also has very strong indoctrination against this kind of thing, but individuals are just as vulnerable as robots to making the wrong decision if they are fed false information. Such as endless attacks on the legitimacy of the president's election, birth certificate, allegations of being a foreign agent, etc.
The US is of course not above assassinating foreign leaders, conspiring to have them deposed, or providing arms to military coups against democratically elected governments. Allende is probably the biggest example.
Edit: and of course there's always the risk of a single rogue individual, from Dr. Strangelove's " Brigadier General Jack D. Ripper" to the very real Ford Hood massacre. In fact, Dr. Strangelove should be required viewing on this subject; not only is it darkly hilarious, it's all about setting in motion destructive partly-automated systems which cannot be countermanded.
What happens when leaders know that, no matter what, never are going to be "targeted" by their own automatic military?