1887 Hanover Agreement
The 1887 Hanover Agreement was a treaty signed between the United Kingdom and the German Empire. Ratified just a few days prior to the marriage of the then Prince of Wales and Princess Sophia of Prussia, the treaty sought to reconcile British-German relations which had been particularly strained due to the issues concerning the Kingdom of Hanover's sovereignty, which arose following the reunification of the United Kingdom and Hanover under King William V.
Despite being formally ratified and generally accepted by both the British and German governments, the treaty was abruptly nullified the next year by the new German Emperor Wilhelm II whom, upon succeeding his father Frederick III, a main architect of the treaty, considerably distrusted the terms and solutions contained in the treaty, deeming them to be "disadvantageous" to German interests. Consequently, the later Duke of Clarence and Avondale, whom was initially stipulated to be the new sovereign of an independent Hanover, as per the terms of the treaty, never ascended to the Hanoverian throne as such a decision was not required following the treaty's nullification, compounded with the British government's view that such a decision would only threaten Britain's national security.
Background
In 1837, following the death of the childless King William IV, the Kingdom of Hanover briefly separated from its century-long union with the United Kingdom due to the accession of the late king's niece Victoria whom, by Hanoverian law was explicitly barred from ascending to the throne in Hanover itself in the case of a living male relative, which led to Prince Ernest Augustus to be crowned King of an independent Hanover instead. However, just a month into his rule, the unexpected birth of the later King William V, a natural descendant of William IV himself prompted calls for reunification with Britain by some of the kingdom's ministers, though this effort remained generally futile during much of Ernest Augustus's reign.
Nevertheless, following the former's death in 1851, a vote was held in the Hanoverian Parliament which decisively favoured a reunification with Britain under its new sovereign. While the outcome was generally accepted in Britain, it drew a starkly negative reception however, particularly in Prussia whose king, Frederick William IV had been somewhat in favour of a united Germany free of foreign dominance, a scheme ultimately frustrated by Hanover's reunification with the United Kingdom.
Otto von Bismarck's Chancellorship
With the appointment of Otto von Bismarck as Minister President of Prussia by William I of Prussia, prospects of a united Germany once more rose to the forefront, in which Bismarck in particular had envisaged a united Germany that would specifically exclude Prussia's main contender and traditional rival, the Austrian Empire. In this, Bismarck himself nevertheless succeeded in his endeavours, though this was immediately frustrated by the continued existence of Hanover which, in contrast to the other German states in Europe prior to the Prussian-led unification was the only German state not to have its own independent ruler after its parliament had decisively voted in favour of a reunification, which meant that any military action taken against the Kingdom of Hanover would've generally meant war against the United Kingdom whose Royal Navy would've easily triumphed over Prussia's.
While Bismarck, now the Chancellor of Germany himself remained committed to his series of foreign manuevers meant to diplomatically isolate the new French Third Republic from the rest of Europe, he also remained firm on addressing the issue concerning Hanover's sovereignity. Initially, his support for the kingdom's rival claimant Prince George of Cumberland, whom was the only descendant of the late King Ernest Augustus proved fruitless as the former was largely unable to overcome the popularity of the British King William V. Subsequent meetings with the Prime Ministers William Ewart Gladstone and Benjamin Disraeli also proved inconclusive as neither men were willing to acede to Bismarck's demand for a German-controlled Hanover, while the latter in return refused to acede to an independent Hanover instead as a compromise.
Treaty
In 1887, under the "splendid isolation" policy championed by Prime Minister Lord Salisbury, a renewed attempt at negotiation with Germany was successfully achieved. With overwhelming support by both the British and German sovereigns, King William V and German Emperor Frederick III respectively, a successful agreement was struck between the two leaders, which stipulated that in the case where the subsequent marriage of the then Prince of Wales and Princess Sophia of Prussia was to first produce a male heir whom by convention, would later succeed his own father to the British throne, then the said heir's eldest younger brother was to then ascend to the throne of an independent Hanover instead upon coming of age, with the later choice of either remaining independent or to place the kingdom under German overlordship, while in the case where the couple's marriage was to first produce a female heir, then the eldest male children from the couple's marriage would then subsequently accede to the Hanoverian throne due to the Semi-Salic Law practiced in the kingdom.
In its aftermath, in the United Kingdom at least, the treaty was largely seen as a positive step towards ensuring Britain's exclusiveness from the affairs of continental Europe while in Germany, it was seen as a resounding success of Bismarck's foreign policies which further strengthened his schemes of engineering a diplomatic isolation targeted towards France.
Aftermath
Despite the treaty being largely approved of by both the British and German governments and public, it was abruptly nullified around a year later with the rise of the new German Emperor Wilhelm II whom had deemed the treaty as being "disadvantageous" to German interests due to the fact that the hypothetically independent King of Hanover was given free rein in determining his kingdom's independence, which meant that a total unification of Germany was not entirely assured by the treaty's solutions. As a result, following the birth of King William V's second eldest male grandchild, namely Prince Frederick, Duke of Clarence and Avondale in the year 1900, the latter was never prepared for the role of an independent King of Hanover as the treaty had previously stipulated, which was largely due to Wilhelm II's own nullification of the treaty and the then ongoing naval arms race between the two countries which meant that the secession of Hanover from Britain would've severely limited Britain's wartime capabilities as the kingdom's territorial limits, which were in close proximity to the German capital city of Berlin would allow British troops to almost directly threaten the German capital in an event of war.