Acts of War

Guy de Lusignon: Give me a war.
Reynald de Chatillon: That is what I do.
--Kingdom of Heaven

In the context of multinational geopolitics, an act of war is aggression done by country A on one or more other countries B of such magnitude that B declares that A's violence exceeds a threshold that justifies reciprocal violence by B toward A. 

While B's reciprocal violence, if taken, is often framed as 'self-defense,' it very well may be offensive in nature. In fact, A and B may engage in ongoing conflict where each side exchanges offensive salvos and defensive parries during the campaign.

Clearly, claiming something to be an act of war is subjective. How much damage must be done, and how many casualties must result before belligerent behavior is deemed an act of war?

Moreover, does a shot even have to be fired? After all, there are surely ways to do violence on others without discharging a weapon directly at them.

For instance, trade sanctions seek to hurt people of a country economically. That economic strife may maim or kill people--both in the country that is the target of sanctions and in other nations as well. 

Arms may only be involved to the extent that they need to be brandished to enforce the sanctions. Quite ironically, those guns often must be flashed at the sanctioner's own citizens to remind them of the consequences of trading with the enemy.

Although they almost certainly regard themselves as 'peaceful,' countries that levy trade sanctions are acting aggressively toward others--and toward their own people.