Occupation: Occupation occurs when a foreign military force establishes control over another nation's territory for an extended period either temporarily (during war) or permanently (through annexation).
Aggression: Aggression refers to any unjustified use of force by one state against another beyond the boundaries allowed by international law. It includes acts like invasions and attacks.
Sovereignty: Sovereignty is the supreme authority of a state within its own borders. Invasions challenge the sovereignty of the invaded country as it loses control over its territory temporarily or permanently.