The Saxe-Coburg prince was elected king by the Congress (June 4); and in redemption of their undertaking the Conference promulgated (June 26) the preliminary treaty, generally known as the Treaty of the XVIII Articles. By this treaty the question of Luxemburg was reserved for a separate negotiation, the status quo being meanwhile maintained. Other boundary disputes (Maestricht, Limburg and variousenclaves ) were to be amicably arranged, and the share of Belgium in the public debt was reduced. Leopold had made his acceptance of the crown depend upon the assent of the Congress being given to the Treaty. This assent was given, but in the face of strong opposition (July 9); and the new king made his public entry into Brussels and took the oath to the Constitution twelve days later. On the same day (July 21) the Dutch king refused to accept the XVIII Articles, declaring that he adhered to the protocols of January 20 and 27, which the plenipotentiaries had themselves declared (April 15) to be fundamental and irrevocable. Nor did he confine himself to a refusal. He declared that if any prince should accept the sovereignty of Belgium or take possession of it without having assented to the protocols as the basis of separation he could only regard such prince as his enemy. He followed this up (August 2) by a despatch addressed to the Foreign Ministers of the Five Powers, announcing his intention "to throw his army into the balance with a view to obtaining more equitable terms of separation."
These were no empty words. The facile success of the Belgian revolution had led to the Dutch army being branded as a set of cowards. The king, therefore, despite a solemn warning from the Conference, was determined to show the world that Holland was perfectly able to assert her rights by armed force if she chose to do so. In this course he had the whole-hearted support of his people. It was a bold act politically justified by events. Unexpectedly, on August 2, the Prince of Orange at the head of an army of 30,000 picked men with 72 guns crossed the frontier. The Belgians were quite taken by surprise. Their army, though not perhaps inferior in numbers to the invaders, was badly organised, and was divided into two parts - the army of the Scheldt and the army of the Meuse. The prince knew that he must act with promptness and decision, and he thrust his army by rapid movements between the two Belgian corps. That of the Meuse fell back in great disorder upon Liege; that of the Scheldt was also forced to beat a rapid retreat. Leopold, whose reign was not yet a fortnight old, joined the western corps and did all that man could do to organise and stiffen resistance. At Louvain (August 12) he made a last effort to save the capital and repeatedly exposed his life, but the Belgians were completely routed and Brussels lay at the victor's mercy. It was a terrible humiliation for the new Belgian state. But the prince had accomplished his task and did not advance beyond Louvain. On hearing that a French army, at the invitation of King Leopold, had entered Belgium with the sanction of the Powers, he concluded an armistice, by the mediation of the British Minister, Sir Robert Adair, and undertook to evacuate Belgian territory. His army recrossed the Dutch frontier (August 20), and the French thereupon withdrew.
The Ten Days' Campaign had effected its purpose; and, when the Conference met to consider the new situation, it was felt that the XVIII Articles must be revised. Belgium, saved only from conquest by French intervention, had to pay the penalty of defeat. A new treaty in XXIV Articles was drawn up, and was (October 14) again declared to be final and irrevocable. By this treaty the northwestern (Walloon) portion of Luxemburg was assigned to Belgium, but at the cost of ceding to Holland a considerable piece of Belgian Limburg giving the Dutch the command of both banks of the river Meuse from Maestricht to the Gelderland frontier. The proportion of the debt was likewise altered in favour of Holland. King William was informed that he must obtain the assent of the Germanic Confederation and of the Nassau agnates to the territorial adjustments.
These conditions created profound dissatisfaction both in Belgium and Holland. It was again the unhappy Luxemburg question which caused so much heart-burning. The Conference however felt itself bound by the territorial arrangements of the Congress of Vienna; and Palmerston and Talleyrand, acting in concert throughout, could not on this matter overrule the opposition of Prussia and Austria supported by Russia. All they could do was to secure the compromise by which Walloon Luxemburg was given to Belgium in exchange for territorial compensation in Limburg. Belgian feeling was strong against surrendering any part either of Luxemburg or Limburg; but King Leopold saw that surrender was inevitable and by a threat of abdication he managed to secure, though against vehement opposition, the acceptance of the Treaty of the XXIV Articles by the Belgian Chambers (November 1). The treaty was signed at London by the plenipotentiaries of the Five Great Powers and by the Belgian envoy, Van de Weyer, on November 15, 1831; and Belgium was solemnly recognised as an independent State, whose perpetual neutrality and inviolability was guaranteed by each of the signatories severally[13].