What was the name of the treaty that the Germans broke when they went

through Belgium to invade France?

1839 Treaty of London

Explanation:

The 1839 Treaty of London recognized Belgium as an independent and neutral state. Until 1911, Belgian strategic analysis anticipated that if war came, the Germans would attack France across the Franco-German border and trap the French armies against the Belgian frontier, as they had done in 1870.

Treaty of London in 1839