Well, I never saw the advantage of a mecha, really. Unless you are fighting gigantic space creatures. Besides, the real advantage is in not having a pilot, or a pilot that could simultaneously command a squad of robots.
I really just pictured the Honda robot with a gun, but more likely something on the order of a four legged or wheeled robot that could do some really limited decision making on its own, such as target acquisition and independent movement, given a destination. Someone else would make most other decisions, such as to when to open and hold their fire. I am visualisng something like a really stupid version of a Tachikoma (and probably a whole lot less cute).
Both of the problems of autonomous navigation and target acquisition are much closer to being solved than they were even just 2 years ago. But, as far as I know, the closest that we have come to perfecting them are easily-fooled vision systems, and cars that can get fairly far while off-road without getting lost. I think that if a world power such as Japan placed themselves on a wartime footing, and were devoted to the development of these technologies, it would be solved much more quickly.
But then again, this makes me think of the difficulty of desiging an interface that would be both powerful enough to make the damn things effectve, and simple and streamlined enough that you could update what you wanted them to do real-time. So, maybe I am thinking of something that is still way too far into the future to worry about what country could accomplish it.
But I bet if Japan were in a race with another country to do it, they would do it first. That would go a long way twoards being able to “bring it” for a country with a small population compared to some of the other countries they have been lined up against in this thread.
WRT nukes, I dunno if Japan’s populace would ever allow them to develop them, much less use them. Unless, of course, they thought they were about to be bombed with another one.
(As a totally unrelated aside, it would be terrible if we allowed them to be attacked by a nuke from Korea, considering we forced them into the pacifist constitiution after beating them with a nuclear weapon. I wonder how the Japanese people would feel about nukes at that point.)