War is Armed Conflict between two nations.
Civil war is armed conflict between two factions within a nation.
War is technically illegal.
It is illegal to use aggressive force against any nation for the purposes of imposing your will onto their people.
Armed conflict is not illegal in self defence.
It is legal for a nation to wage war in self defence.
In Law an individual is allowed to use reasonable force in self defence.
In war it appears that any amount of force is reasonable in the pursuit of national security and national defence.
All modern Armies have been renamed Defence Forces, in order for them to be legal.
A pre-emptive strike is defined as an act of self-defence, even though it is a premeditated and an aggressive act of killing people.
Whenever an ego wishes to exert its will over another there will be conflict and conflict inevitably leads to war.
A nation does not have an ego. It is the ruling elite of a nation that exercises the will of their ego self.
That same ego decides what is legal and what is legitimate.