Sabotage, border clashes and airstrikes happen all the time without constituting acts of war that give casus belli to their target. A basic legal condition of a just war is that it is proportionate and embarked upon as a last resort: launching a war because a dolphin you trained was killed is neither of those, as sad as it might be.
Sure, things happen all the time without a war breaking out. But it's not like you can "just kill" or "just attack" without repercussions. That's ridiculous thought. It's similar to saying "what's the reason to have a security guard in the shop, when you can just take a pistol and kill him". Security isn't about having an impenetrable defense; such a thing isn't possible. Anything can be destroyed in the end.
There's always a repercussion for attacking the military, be it diplomatic, economic or a military retaliation.
See, for example, the OED reference:
'An act by one nation intended to initiate or provoke a war with another nation; an act considered sufficient cause for war.'
https://www.oxfordreference.com/view/10.1093/oi/authority.20...