This question has been answered a few hundred years ago by the strategic genius Sun Tzu.
One of his most famous quotes is relevant to this question:
"If you know the enemy and know yourself you need not fear the results of a hundred battles."
Knowing who your enemy is, where his troops are stations, what tactics he uses, what plans he has, how he leads his military and people.
Especially in the past but still valid to this days are spies and saboteurs who often physically go into enemy territory in order to gather information, sabotage important processes or assassinate important people.
Even though the old fashioned spies and saboteurs of course still exist but in this time where information is shared trough computers over networks it's the most obvious systems for spies and saboteurs to use and we coined these specialists "Hackers".
They still have the same basic function of gathering information, disrupting the enemy or (politically) assassinate someone important by leaking information.
Hackers are tools of war and very crucial ones at that and countries that invest in these spies will have an major edge over other countries that don't.
If played well even the smallest countries or even individual groups and people can start and end a war.
In the end war is nothing more then a tool used to further a political agenda, it always has been.