I don't really consider this "an act of aggression" though, not when you consider the scope of what the Horde is so avidly collecting: Azurite, the most potently volatile substance in the known realm. The Horde is actively collecting a volatile substance, and are expecting that any involvement would be hostile. Which, no shit, if you were collecting the reagents for WMDs, you should expect a harsh reprisal from other nations. The simple act of collecting it would be a severe threat to every other nation and a massive "act of aggression" by itself, before the Alliance is even involved. The shear comparison from "collecting resources to build WMDs" to "Rigging construction equipment with explosives to send a message to stop digging for volatile resources" are VERY different things.
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You are ignoring the fact that Azerite is extremely volatile, the most volatile substance in the known realm. In your example, the assassination of China's government officials would have prevented them from illegally orchestrating the trade of the highest grade of brand new WMD resources, which is illegal and breaks the terms of treaty to begin with. From a worldview perspective, those that ordered the assassins would be seen as heroes that prevented WW3 by preventing a new age of WMDs to be built.