The final coalition's declaration was directed against Napoleon Bonaparte personally rather than against France. Britain and its partners still regarded themselves as allied with the restored French king and government. Naming Napoleon as the enemy let them present the campaign as support for legitimate France. The legal wording therefore helped turn another invasion into an action against a usurper instead of an acknowledged war of aggression.
Notable Point · 30:26-30:58
Why the Allies declared war on Napoleon personally
The Hundred Days coalition targeted Napoleon rather than France so its members could claim to defend the restored French government rather than invade it.