The Kingdom of Hanover existed from its creation after the Congress of Vienna in 1814 (being a duchy and electorate within the HRE prior) until its annexation by Prussia after it lost the German-German war on the Austrian side. The Prussian crown annexed the kingdom and organized it as the newly formed Prussian province of Hanover in 1866. Hanover was notable for the fact that the British monarch came from the House of Hanover starting with George I in 1714 and ending in 1837 with the death of William IV.
My question is: How was it possible that Prussia was simply allowed to annex Hanover in 1866? Hanover was the 4th largest state in the former HRE region, as a kingdom a considerably prestigious country, and by providing the British monarch for more than 100 years certainly not a diplomatic leightweight.
Was there no diplomatic outcry, no foreign protest towards such an annexation? It would seem to me that an annexation of such central German power by an already nascent Prussia in the second half of the 19th century would certainly not just have been accepted by the other Great European Powers just standing by? However, I have trouble finding sources in literature to analyse reactions in Europe at the time. Perhaps somebody can enlighten me?