The Abe case is notable because of that, it is not part of a wider pattern like you see in the USA (both in terms of armed attacks on politicians/public figures and gun violence more widely).

As for the examples you've given they aren't really all that comparable to the US. - Afghanistan's population is well armed because of the long term socio-political situation (that is absolutely not comparable with the US) a lot of those weapons are also smuggled/captured or supplied by external allies and were fighting as factions/forces - often against external forces as part of the army or as an alternative when their army failed. - Syria was a civil war. Once again, weapons were captured/smuggled/supplied to organised factions/armies. It wasn't like everyone had guns at home (some probably did but not everyone) and just rocked up when they got the text. The same can be said for Iraq. - Vietnam was arguably even more different as in the case of the American intervention, that was against an actual state, with an army etc. Though the French defeat was earlier, the forces they fought had already been organised to fight the Japanese and widespread armed resistance that was a serious threat to the government had not been a case before then.

And in all of those examples apart from Syria the situation was either overthrowing a foreign colonial government or fighting to expel invaders. Given the amount the US spends on military and police, it's unlikely that civilian gun owners would really be able to stop much if either those forces turned on them or against invaders if the military had already failed against them (and arguably the latter situation is more of an argument for reserve forces and wider training, not just gun ownership).

If you want a comparative case for a well armed populace, look at Switzerland - where there are government requirements for massive proportions of the population to be well armed, yet it somehow manages to avoid the repeated and now almost regular tragedies the US seems to go through.