This cartoon was published in 2005, on the 60th anniversary of Hiroshima, but I only just saw it today. A boy, who must have been born in the 90s, given the publication date of the cartoon, sits on the lap of his grandfather (or father, but I'll just call him a grandfather) and sees a huge explosion on television. The biography of such a boy would make such a question completely natural: it's quite likely that he would associate films of explosions with terrorism, so of course he might well ask his grandfather, "which terrorist group did that?" A boy at that time would have little reason to have the Second World War as a frame of reference, and even less reason to really be able to give much meaning to such expressions as "Pearl Harbor," "Holocaust," or "Hiroshima." His question is an attempt to understand what he sees in terms of what he knows.
The grandfather would certainly understand the question quite differently than that, though, especially if we assume that the man depicted here is an American. After all, he would then most likely see the dropping of the atomic bomb on Hiroshima in the conventional fashion: it was a necessary step to force the Japanese to surrender; the alternative would have been a horrible war of attrition, with deaths at least an order of magnitude more than those caused by the atomic bombs dropped on Hiroshima and Nagasaki. Without addressing the debates about the historical accuracy of these tactical and strategic considerations (debates among historians who have looked at the primary sources), this grandfather would perhaps try to explain to the boy that this explosion was not an act of terrorism because it was part of a war. Explosions in wars are not terrorism, he might explain.
A 2005 reader of the cartoon might see the humor in it in such terms: a boy interprets what he sees in terms of what he knows, and his grandfather explains the not entirely straightforward distinctions to him. But such a reader might also respond to the boy's question without taking the rest of the picture into consideration. Such a reader might well get very upset about how the picture opens up the possibility of equating Hiroshima (a legitimate action as part of a formally declared war) and 9/11 (a violent act of what is legitimately called terrorism). Alternatively, a reader vehemently against the Iraq War might appreciate how the raising of this possibility calls the legitimacy of war in general and particular wars into question, even though such a reader would likely still consider the Second World War a just war (and even fully agree with the interpretation of Hiroshima that the grandfather might use to explain the situation to the boy).
Seeing the cartoon in 2013, I can see all these possible layers in it, but I also see it from the perspective of the current military actions of the United States. Whatever completely legitimate distinctions might be made between them, Hiroshima and 9/11 share at least one feature: they were attacks directed against civilians. The current American government justifies its ongoing missile attacks in Pakistan, Yemen, and elsewhere as part of a "War on Terror" and hence as militarily legitimate. The Bureau of Investigative Journalism has a web page that summarizes known attacks using drone-based missiles in such countries; their reporting makes clear that the attacks are not solely directed at "military" targets that are part of the "war on terror." They also provide extensive evidence of the apparent American policy of attacking rescuers and funerals, a tactic which the American government justifiably condemns when it is used by those it calls "terrorists."
Given this context, the cartoon has an additional layer of meaning today: the current military actions undertaken by the United States are much harder to distinguish from "terrorism" than the bombings of Hiroshima and Nagasaki are. In fact, many of them are indistinguishable from terrorism, and their straightforward effect is not to defeat "terror" through a supposed "war on terror," but actually to generate more potential terrorists. Today, a well-informed grandfather asked such a question by a clever grandson might find it much harder to clarify to the boy how it is that military actions taken by the United States are not "terrorism."
There's an easy way for the United States to get out of the trap it has put itself and its grandfathers in. It could do what every other country that has been faced with terrorist attacks this century has done: it could use the criminal justice system to pursue and prosecute terrorists. It could demilitarize its response to "terrorism" and use the rule of law — again, as every other country has done — to bring the criminal perpetrators of terrorism to justice — justice, and not vengeance.