The Treaty of London
Historically the dutch king didn't acknowledge Belgium as an independent country until he signed the treaty of London in 1839. Belgium became independent in 1831 by a revolt. Then they were defended by France and United Kingdom and the dutch king was forced to sign the treaty.
In the game when the treaty is signed, Netherlands will lose all of its cores in Belgium. Then it will get 10 prestige and good relations with all the other European nations. The treaty can only be signed if the Netherlands have over 100 points of relations with United Kingdom or UK has over 50 influence in Netherlands.
While Belgium's de facto independence from The Netherlands had been established by force of arms during the Belgian revolution of 1831, it had not been confirmed de jure by The Netherlands until the signing of the Treaty of London. In 1839 The Netherlands, along with all the Great Powers of Europe, signed the treaty, which formally established Belgium's independence and neutrality. By signing the treaty we will lose our claim to Belgium, but our standing in the eyes of the world will improve.