Short answers, pulled from the story.
Alfonso XIII left Spain voluntarily on the 14th of April 1931 after municipal elections on the 12th of April were interpreted as a plebiscite on the monarchy. The Republican coalition won sweeping majorities in major cities, and the head of the Civil Guard told government ministers the Armed Forces could not be relied upon to sustain the monarchy. The Second Spanish Republic was proclaimed the same day Alfonso departed.
Alfonso XIII received a nomination for the Nobel Peace Prize in 1917 for his work establishing a wartime office in the Royal Palace of Madrid that assisted prisoners of war on all sides of World War I. The prize that year was awarded to the Red Cross. He remains the only monarch known to have received a Nobel Prize nomination.
The Battle of Annual was one of Spain's worst military defeats during the Rif War in Morocco. Alfonso XIII had sent his favoured general Manuel Fernandez Silvestre a telegram reading "Hurrah for real men!" urging him not to retreat, and Silvestre pressed forward into catastrophe. A 1922 Cortes investigation found evidence that Alfonso had personally encouraged the advance, and his response on learning of the defeat - shrugging and saying "Chicken meat is cheap" while playing golf in France - became notorious.
On the 31st of May 1906, Catalan anarchist Mateu Morral threw a bomb at Alfonso XIII's wedding procession in Madrid, killing thirty people and wounding one hundred others. Alfonso and his new wife Princess Victoria Eugenie of Battenberg survived because their carriage was lined with bulletproof material developed by the Polish inventor Jan Szczepanik.
Alfonso XIII fell ill during the 1918 influenza pandemic. Because Spain was neutral in World War I and had no wartime censorship, his illness and recovery were reported openly to the world, while outbreaks in belligerent nations were suppressed. This created the false impression that Spain was the most affected country and led to the pandemic being called the Spanish Flu.
Ratoncito Perez, the Spanish-speaking world's equivalent of the Tooth Fairy, first appeared in a tale written by Luis Coloma in 1894 for Alfonso XIII, who had just lost a milk tooth at age eight. The king appeared in the story as "King Buby." The tradition of the mouse replacing lost teeth with a small payment while children sleep is now followed across Spain and Hispanic America.