Full Idea
Every violation of an independent state is called aggression, which fails to differentiate between a seizure or imposition, and an actual conquest. …But what they have in common is that all aggressive acts justify forceful resistance.
Gist of Idea
Even non-violent intrusive acts between states count as aggression, if they justify resistance
Source
Michael Walzer (Just and Unjust Wars [1977], 04)
Book Reference
Walzer,Michael: 'Just and Unjust Wars' [Penguin 1984], p.52
A Reaction
[compressed] Walzer concedes that this makes 'aggression' rather imprecise, and small acts can be used as an excuse for desired violent resistance. Each entrant in August 1914 seems to have had a slightly different motive.