Just finished The Lost History of 1914: Reconsidering the Year the Great War Began (Walker and Co., 2012). It recounts a variety of episodes and events that were roiling each of the major countries in the months before they plunged into World War 1, but that were largely forgotten in the aftermath of the war. Those events were, so to speak, overshadowed by the glare of that terrible war and largely lost to the memory of those who lived after. England was dealing with near civil war over revolt in Ireland, America was preoccupied with Pancho Villa and a revolution brewing in Mexico, France was obsessed with a political scandal that derailed the career of a figure who might have argued for peace not war in 1914, and Germany's Kaiser was trying to manage a Reichstag full of feisty Social Democrats. When the heir to the Austrian crown was assassinated in Sarajevo in August, the countries tumbled into war, but it could have played out so differently. For anyone who has read Tuchman's The Guns of August, this book gives a whole different perspective on the events of 1914, both those we remember and those we have forgotten.
I suspect the phenomenon is not limited to 1914 and the Great War. Any similar great event is likely to overshadow and render almost irrelevant (at least in memory) surrounding events. A good example in LDS history in 1844 and the assassination of Joseph Smith, which not only dismayed tens of thousands of LDS members, but also triggered an unprecedented leadership crisis: who should now lead the Church? Joseph Smith could not be replaced, but should his position as President, with two counselors in a First Presidency, be filled? (It wasn't.) Should the remaining counselor in that presidency now preside? (He didn't.) What weight should the remaining councils or offices be given? (The quorum of apostles became the ruling quorum. displacing other candidates.)
But what events were lost in the glare of 1844 and its attendant succession crisis? The single issue of the Nauvoo Expositor and its suppression was quickly forgotten, at least in Nauvoo. More generally, the subsequent history of the Mormons in Nauvoo between June 1844 and their departure in early 1846 is rarely given much attention or memory, although it was a key time in which Brigham Young's leadership of the Church (as President of the Twelve, not as President of the Church) was confirmed, the Nauvoo temple was largely completed, and temple ordinances were extended to the general membership of the Church. Had events and politics played out differently, the Saints might never have left Illinois.
Any other candidates for this sort of lost history effect? The years 1857 and 1890 come to mind for LDS history, but I can't think of any such dates in the 20th century.