I've recently been reading Barbara Tuchman's fantastic book on WW1 The Guns of August and it got me thinking about the idea of preemptive war. One of main rationalizations ascribed to Germany is the idea that they need to attack France preemptively in order to secure victory in the war — even overriding concerns about the repercussions of violating Belgian neutrality.
I've also read Caesar's Commentarii de Bello Gallico and one of his justifications for invading Gaul (paraphrasing from memory) is that Gaul is "civilizing" or becoming organized more like the Roman state. He describes the Aedui as having a strikingly similar political structure to Rome; they elect a consul to a year-long term, the office can only be held once, no family member can hold high office simultaneously etc. The implication is that if Gaul organizes and unifies, it will become a serious threat to Rome and should therefore be dealt with sooner than later.
My question is what are some other examples of preemptive war that you can think of? And, maybe this part of the question is more suitable for philosophy, was it legitimately justifiable by the attacking power?
Source: reddit post