That is a nightmare scenario.
However, it is highly dangerous to the person[s] attempting it. First of all, there would have to be a lot of smuggling in order to get enough devices in enough cities to make a serious impact on a large country outside of immediate fear and panic. One device by itself would not be effective militarily; it would be counterproductive like sowing dragon's teeth.
If one such smuggling were detected, all counter-measures around the planet would be activated. This would include full mobilization. Besides catching the smuggling and the smugglers dead or alive in that one case, the smugglers would be identified, the real danger to the perps.
So, it would require several smuggling attempts to do anything series, and the more attempts, the greater the chance of discovery. Once discovered, begin the endgame.
Why do you think that means that it wouldnt be done? I understand your reasoning but it is far too civilized.
It depends on how you do it and whom you use to do it. You seem to assume that they will be using the traditional method used by people who plan on coming home after the war. They won't. Instead they will find suicidal volunteers to fly planes loaded with gas, bugs or bombs to several targets. If only four or five hit we are talking about potentially millions of lives.
If volunteers are not forthcoming then you just grab a few pilots families. After they watch you cut off a couple of ears they will fly the planes.