When the Prince of Orange assumed the title of William I, Sovereign-Prince of the Netherlands, at Amsterdam, on December 2, 1813, the principal towns were still occupied by French garrisons; but with the help of the allied forces, Russians and Prussians, these were, in the opening months of 1814, one by one conquered. The Helder garrison, under the command of Admiral Verhuell, did not surrender till May. By the end of that month the whole land was freed.

The first step taken by the Sovereign-Prince (December 21) was to appoint a Commission to draw up a Fundamental Law according to his promise. The Commission consisted of fifteen members, with Van Hogendorp as president. Their labours were concluded early in March. The concept was on March 29 submitted to an Assembly of six hundred notables, summoned for the purpose, the voting to be 'for' or 'against' without discussion. The gathering took place in the Nieuwe Kerk at Amsterdam, Of the 474 who were present, 448 voted in favour of the new Constitution. On the following day the Prince of Orange took the oath in the Nieuwe Kerk and was solemnly inaugurated as Sovereign-Prince of the Netherlands.

The principal provisions of the Fundamental Law of March, 1814, were as follows:

The Sovereign shares the Legislative Power with the States-General, but alone exercises the Executive Power. All the sovereign prerogatives formerly possessed by provinces, districts or towns are now transferred to the Sovereign. He is assisted by a Council of State of twelve members, appoints and dismisses ministers, declares war and makes peace, has the control of finance and governs the overseas-possessions. The States-General consist of fifty-five members, elected by the nine provinces, Holland, Zeeland, Utrecht, Overyssel, Gelderland, Groningen, Friesland, Brabant and Drente on the basis of population. The members are elected for three years, but one-third vacate their seats every year. They have the right of legislative initiative, and of veto. The finances are divided into ordinary and extraordinary expenditure, over the former the States-General exercise no control, but a general Chamber of Accounts (Algemeene Rekenkamer) has the supervision over ways and means. The Sovereign must be a member of the Reformed Church, but equal protection is given by the State to all religious beliefs.

It was essentially an aristocratic constitution. At least one quarter of the States-General must belong to the nobility. The Provincial Estates had the control of local affairs only, but had the privilege of electing the members of the States-General. They were themselves far from being representative. For the country districts the members were chosen from the nobility and the land-owners; in the towns by colleges of electors (kiezers), consisting of those who paid the highest contributions in taxes. Except for the strengthening of the central executive power and the abolition of all provincial sovereign rights, the new Constitution differed little from the old in its oligarchic character.

It was, however, to be but a temporary arrangement. It has already been pointed out that, months before his actual return to Holland, the prince had received assurances from the British government that a strong Netherland State should be created, capable of being a barrier to French aggression. The time had now arrived for the practical carrying-out of this assurance. Accordingly Lord Castlereagh in January, 1814, when on his way, as British plenipotentiary, to confer with the Allied Sovereigns at Basel, visited the Sovereign-Prince at the Hague. The conversations issued in a proposal to unite (with the assent of Austria) the Belgic provinces as far as the Meuse to Holland together with the territory between the Meuse and the Rhine as far as the line Maestricht-Dueren-Cologne. Castlereagh submitted this project to the allies at Basel; and it was discussed and adopted in principle at the Conference of Chatillon (February 3 to March 15), the Austrian Emperor having renounced all claim to his Belgian dominions in favour of an equivalent in Venetia. This was done without any attempt to ascertain the wishes of the Belgian people on the proposed transference of their allegiance, and a protest was made. An assembly of notables, which had been summoned to Brussels by the military governor, the Duke of Saxe-Weimar, sent a deputation to the allied headquarters at Chaumont to express their continued loyalty to their Habsburg sovereign and to ask that, if the Emperor Francis relinquished his claim, they might be erected into an independent State under the rule of an Austrian archduke. A written reply (March 14) informed them that the question of union with Holland was settled, but assurances were given that in matters of religion, representation, commerce and the public debt their interests would be carefully guarded. Meanwhile General Baron Vincent, a Belgian in the Austrian service, was made governor-general.