If states cannot agree, war decides.

What is an injury?

You cannot define this exactly. States touch each other in countless ways. A state can put its whole honor into every single detail.

When does a state become touchy?

Especially after a long peace at home. A strong power then likes to look for a field of action abroad. It even creates such a field itself.

Summary: In the end war decides disputes between states. What counts as an injury is not fixed. After a long calm, strong states like to look for a field abroad.