Treaty of London (1867)

The Treaty of London (), often called the Second Treaty of London after the 1839 Treaty, granted Luxembourg full independence and neutrality. It was signed on 11 May 1867 in the aftermath of the Austro-Prussian War and the Luxembourg Crisis. It had wide-reaching consequences for Luxembourg and for relations among Europe's Great Powers.
Effects.
The immediate effect of the treaty, established in Article I, was the reaffirmation of the personal union between the Netherlands and Luxembourg under the House of Orange-Nassau. It lasted until 1890, when Wilhelmina ascended the Dutch throne. As a form of agnatic succession was then in effect in Luxembourg (under the Nassau Family Pact of 1783), the Grand Duchy could not pass in the female line. Instead it was the older branch of the House of Nassau (Nassau-Weilburg, now called Luxembourg-Nassau) that inherited that dignity, giving Luxembourg its own exclusive dynasty.
The Luxembourg Crisis had erupted after French Emperor Napoleon III attempted to buy Luxembourg from the Dutch King William III. Consequently, maintaining Dutch dominance over the de jure independent Luxembourg, free from French interference, was of paramount importance to Prussia.
The neutrality of Luxembourg, established by the First Treaty of London, was also reaffirmed. The parties that did not sign the earlier treaty were to become guarantors of Luxembourg's neutrality (an exception was Belgium, which was, itself, bound to neutrality).
To ensure Luxembourg's neutrality, the (westward) fortifications of Luxembourg City, known as the "Gibraltar of the North", were to be demolished and never to be rebuilt. To the east, the city was protected by a deep river valley and medieval fortifications that still exist. Dismantling the westward and underground fortifications took sixteen years at a cost of 1.5 million gold francs and required the destruction of over of underground defences and of casemates, batteries, barracks, etc. The still very large residual fortifications of Luxembourg City are now part of the World Heritage List of the UNESCO.
Furthermore, the Prussian garrison, which had been sited in Luxembourg since 1815 in accordance with the decisions of the Congress of Vienna, was to be withdrawn.
The Austro-Prussian War had led to the collapse of the German Confederation. Two former members, the Grand Duchy of Luxembourg and the Duchy of Limburg, had the Dutch king as their head of state (as "Grand Duke of Luxembourg" and "Duke of Limburg"). To clarify the position in the wake of the death of the Confederation further, the Treaty of London affirmed the end of the Confederation and stated that Limburg was henceforth to be considered with all its "territories" an "integral part of the Kingdom of the Netherlands". 
The independent Grand Duchy of Luxembourg, still linked to the Netherlands by a personal union, would rejoin the newly re-established German customs union, the "Zollverein", in which it would remain until 1 January 1919, long after the personal union had ended (1890).
Signatories.
The treaty was signed by representatives of all of the Great Powers of Europe:
Italy was originally not invited, but King Victor Emmanuel II persuaded the other kings and emperors to invite his representative. The treaty did not directly affect Italy in any appreciable manner, as she had little relation to Luxembourg. However, it marked the first occasion on which Italy was invited to partake in an international conference on the basis of being a Great Power, and, therefore, was of symbolic value to the fledgling Italian kingdom.