What happened to Louis Napoleon after his second attempt?

The biggest principle that the allies wanted to clean up after Napoleon was legitimacy.

Explanation:

Napoleon was a legitimate, recognized head of state, who everyone except England was allied with at one time or another. As a foreigner, they couldn’t execute the French head of state for acting on behalf of France. To just declare him a criminal and shoot him would have been admitting that the Czar of Russia and Emperor of Austria had been making deals with a criminal.

Also, some of the allies LIKED changes made by Napoleon and wanted to keep it. For example, Kings of the Confederation of the Rhine wanted to keep being Kings, not Grand Dukes or Electors. It was in their interest to not declare Napoleon an outright criminal.

Even the allies holding him on St. Helena wasn’t backed by law. How they were treating Napoleon had no legal precedence. They were making things up as they were going along. The reason why the British would never allow Napoleon to set foot on England was that Napoleon’s supporters would have filed a Habeas Corpus suit on behalf of Napoleon and make the British courts answer what law they were holding Napoleon under.

In the long run it actually played to the advantage of British that Napoleon was alive and under their control. Letting loose Napoleon was the ultimate political trump card they had against the Germans and the French.