No it is literally illegal to invade another country unprovoked, the crime is called “war of aggression”. It is one of the foundational pillars of international law. The Nuremberg judgement called aggressive war the “supreme international crime”. This is the reason US shuns and delegitimizes the ICC and the ICJ, the two bodies supposedly enforcing international law.

It is also separately illegal to annex another country even by mere threat of force, even if no actual force was used.