Dropping normal bombs on it is highly identifiable and attributable, and not very damaging. Also, a normal bomber or cruise missile might actually get shot down

Dropping normal bombs on it is highly identifiable and attributable, and not very damaging. Also, a normal bomber or cruise missile might actually get shot down. There would be no doubt about Article 5 and the need for a military response. Dropping a nuke on it is equally or more damaging, but likewise highly identifiable and attributable, and would force an even bigger reaction, likely even by China. Also, again, might be shot down. An explosion of a cargo ship is just as damaging as a small nuke, but cannot be immediately identified as an attack - even if people suspect it, it's hard to conclusively prove that it wasn't an accident. This makes it hard to muster a unified and strong response.