Yes, it riles up the empire. They don't like it. They certainly regard it as a hostile, aggressive act.
But you're not providing enough military support for the smaller nation to invade and conquer the empire. It makes it more expensive (perhaps prohibitively so) for the empire to conquer the other country. That's only "aggressive" if the empire was intending to conquer the other country, and only because it removed from them an option that they really wanted to use. They don't lose a foot of ground. All they lose is the option of themselves invading.
>But you're not providing enough military support for the smaller nation to invade and conquer the empire.
The fear isn't that the border country tried to invade, the fear is that the country providing the arms will use them as a stockpile to invade. A buildup of arms along the border has been seen as an aggressive provocation for millennia.
Bringing this back to the specific instance at hand: China sees the buildup of arms in Taiwan as a stockpile for the US to invade mainland China? No way. (I mean, yes, they might be paranoid enough to see it that way, but it's completely divorced from reality. There is exactly zero chance that the US would do so. What's much more likely is that they would use it as an excuse to move to "counter US aggression".)
>I mean, yes, they might be paranoid enough to see it that way, but it's completely divorced from reality
It began in ~1950, when it had been less than a decade since the latest of the several times the US has invaded China. It's not unreasonable to worry that the US would lead a new Eight Nation Alliance to return the Kuomintang to power.
It's less likely now, but it could still be used in some kind of proxy war somewhere like Korea. Arming the border does far more than just prevent China from invading.
Yes, it riles up the empire. They don't like it. They certainly regard it as a hostile, aggressive act.
But you're not providing enough military support for the smaller nation to invade and conquer the empire. It makes it more expensive (perhaps prohibitively so) for the empire to conquer the other country. That's only "aggressive" if the empire was intending to conquer the other country, and only because it removed from them an option that they really wanted to use. They don't lose a foot of ground. All they lose is the option of themselves invading.